You’d get such few arcanes from event keys (if they have the same rate as now, just color restricted) that you’d basically just be throwing away your keys. Plus, the wait between events could be as long as six months, whereas most glory packs should cycle in less time. I highly doubt given the option that you’d want to “try” for an arcane through an event chest with their abysmal drop rate, and I’d rather not have increased drop rates on stones taking up valuable troop spots.
Anyway, my current droprate recorded for explores is 100/1717. My last recorded set indicated 69/1111. So, between ~5.8% and 6.2%, which would put them at about 1 / 17, on average. If using a decently fast team, even with soul farming, you should be getting in about 40-60 battles in per hour. My fastest non-soul team team averages 45 seconds from map to map, which is about 80 battles per hour, but requires pretty high end stuff to do that. They really aren’t all that hard to obtain, except maybe your first few, where you have to go a bit slower given you wouldn’t have anything traited or possibly even fully leveled yet.
If you want consistent drops, you need volume, and if you want volume, you need speed (or a lot of time). Just keep at it.