One thing I haven't seen considered here is that you can get Epics that don't belong to the current event kingdom. While recording my first 300 event keys on Silverglade week, in addition to epics for the kingdom I also got:
For the kingdom itself, I got 7 Elspeth and 17 Silvermaiden - actually higher than your 6.5% estimate for just kingdom droops, but also extremely skewed toward the Silvermaiden. Sometime in the next 150 keys, which I did not record each instance of but have a bunch of screenshots around the time, I obtained 3 more Elspeths and 3 more Silvermaiden, still leaving Silvermaiden way way in the lead on overall drops. So, in practice, I spent 450 event keys for this milestone and it put Elspeth just within reach. This was roughly in line with my original estimates of 350 to 500 event keys, without going hard into the math, and just based on my experiences with the relative drops in Blackhawk. In addition to this, I ended up spending around 2500 glory keys and 28500 gold keys to fill out the other requirements. This is pretty much on point with your common/rare estimates above and my initial estimates as well.
So heres where it all breaks down.. In my experience, the event key drops are also at least somewhat weighted toward the "you can win" (and possibly extending to the other troop that can be purchased with glory), despite other troops in that same rarity tier being in the kingdom. This one is mostly anecdotal, however, since I don't have any hard data to back it up. For myself, during Glacial Peaks event I got a lot more Yeti than Frost Wolf; during BlackHawk, I got many more Bonnie Rose than Lil' Johnny Bronze before even purchasing any glory packs; and during Silverglade, I ended up with 20 Silvermaiden (again, before purchasing any from glory packs) by the time I had 16 Elspeth - and two of those Elspeth game from glory keys, one from the quest, and three from snotstones, so I essentially got twice as many Silvermaiden from event keys. I unfortunately did not record the exact number of Keys to get the ratio of Incubus to Twisted Hero dropped when hunting for Dark Monolith, so I'll have to collect more data when the next legendary hits. Could just be bad luck on my part, but its too much of a trend to just ignore. I'll be paying special attention to the drops when the next legendary and next kingdom roll out for sure.
Ultra-rares can also come from other kingdoms, but you'll have so many more of them by the time you finish legendaries and epics that they should never be the bottleneck. While I was recording, I noted the only ultra-rares I got that that did not belong to the kingdom were Druids, Treants, and Yeti, getting more Druids and Treants than Yetis (probably a coincidence in this case), but actually has me wondering what methodology there is to "other" troops in kingdom chests, but that is a discussion for another time. Regardless, going back and looking at the footage, 52 of the ultra-rare drops in my initial 300 keys were not from the current kingdom, while 187 were either Enchantress or Unicorn. Still an over 60% droprate for your target kingdom, and even if the kingdom releases with three ultra-rares and only one epic/legendary, you'll still have way way more than you need by the time you complete legendaries and epics.
On the resource side, our guild pulls away from Glory Keys in favor of always finishing the others, so my strategy is a bit different here. I'm on the high side of tribute collects, so I have plenty of gold keys to finish every other kingdom without spending any gold, which I can decide if/when I see how good of a spread I got from just event keys. Tributes account for at least a third of my monthy gems, which is also left out of the calculation. I use almost exclusively gems on VIP chests when attempting mythics, because it is twice as efficient as any other resource, trying to save the others for new kingdom releases, and only bother with other keys if I really want the mythic. Event keys also can be supplemented by the 60 revenge battles, or buying then with 180 glory as well, which, going forward, is what I will be doing if I ever run into that wall rather than buying them with gems. Barring new kingdom rares, which I am going heavier on Gold Keys anyway, chances of getting something I need in 9 glory keys is much smaller than the chances of getting something I need in 1 event key.
As far as event keys many event keys are going to be eaten up by upcoming event legendaries, though, this is going to heavily eat into that supply. On average, you'll need 100 more event keys per new legendary, but spitting out 300 for a single copy is not unheard of, and this will be even worse when they start adding third legendaries. There is often at least one legendary between new kingdoms, in this case, two. If we tack this onto the cost, we'll need to earn 200 more event keys between kingdom releases somehow.
I'm still sticking to my estimate of at least 450 event keys for just the kingdom, plus accounting for an average of 100 spent every time a new legendary pops up. At this point, it would be extremely foolish not to prioritize getting at least one of each new event legendary if you have the resources to do so given how bad the drop rate is when they hit the general drop pool (not to mention the four week wait).
So overall, adjusting for resources I personally find easier to get, my estimates are:
~650 event keys (450 or equivalent saved going into the event week)
~2000 glory keys (my guild and PvP tiers and glory pack purchases get about 250 a week)
~30k glory keys (guild, tributes and number of glory packs purchased puts me at 2k or more gold keys per week in total)
Or a weekly total of:
81.25 event keys
250 glory keys
3750 gold keys
Which means, beyond what I get every week, I'd still need per week:
39.25 event keys - 529-588 gems (depending on bought in bulk or not) OR 7065 glory, or some combination of both (slightly less with event rewards, revenges, and extra levels). This is less glory than it sounds like, especially if you shy away from buying glory packs on weeks where you get the troops you need incidentally in the process hunting for a new legendary. Spending an average 5500 glory per week when there is not a legendary in the event key drops and zero when there is leaves a total of 33,000 glory for the 8 week period spent on glory packs, or 4125 per week. This leaves a total value of 11190 glory to collect per week and still break even toward this goal, without needing to spend a single gem toward event keys. This is most certainly possible with full seven star kingdoms, maxed statue bonuses for tributes, frequent collects for tributes, glory from PvP, guild, and opening the 20k+ gold keys at the end of your eight weeks, without even dipping into legendary tasks.
0 extra glory keys (burn everything you have for chances at new kingdom troops when it hits, you want to avoid this drop pool as much as possible)
1750 gold keys - ~525k gold (this number can be adjusted down if more glory keys are obtained, or guild keys are used to supplement while at 20k seals, or just by waiting a bit longer for more gold keys). If I were getting every guild task every week, as with many that would even attempt this, this "extra" gold needed here would be in the realm of only 100-150k, still without having to supplement glory into glory keys. Chances are, if you are doing this every kingdom, you'll still have to stock at least this much gold per week on average to ensure you reach the goal for just commons, though, at least until nine star kingdoms are possible and prevalent. And that is assuming nine star comes before enough troops have been added to significantly increase the number of gold keys required.
And of course still luck. Lots of luck.
If event keys are supplemented with glory, this still allows for every gem and gem key obtained to hunt for mythics. Otherwise, (with no legendary tasks) you'd only be able to afford <50 VIP keys a month along with whatever gem keys you saved. Some people have had luck with glory keys dropping mythics, but the stated relative droprates are not in your favor to use glory keys for this purpose, and gem keys don't have the volume to significantly contribute to new kingdoms, nor can they drop rares, and you'll have plenty of Ultra-Rares regardless.
All this makes it unlikely to get every event legendary and still have something left to hunt for mythics after just finishing Silverglade in the first week as well with "only" completing every single guild task every week and collecting tributes 16 times a day. I therefore probably wont bother completing the next one right away unless the skill bonus is good, the troops are decent, and my gold key numbers are close enough to make the common/rare bottleneck possible within a month of the kingdom releasing or with whatever gold I have on hand. Still definitely doable though through careful planning even without Legendary Tasks, if I played enough, so those that do legendary tasks should be able to plan for it easily. If mythic ascending commons are not possible, though, I have no reason to spend more than a few hundred gold keys (unless the troops are exceptional), because I can then get Suncrest AND the next kingdom's commons to mythic in the same week, and be "caught up" on every second kingdom release rather that always being two months behind.