Had the game ages but never really played it, found it again and I’m really enjoying it!
So Q1, The subscriptions in-game shop are all 15 days, is there a way of getting all of them with one purchase or similar?
Q2, Changing Home Kingdom, do you have to complete the quests on the city you want to make it tenable?
Hmm, did some google-fu, I think I’m going for the DK armour first and then go with the occasional impulse buy 
Home kingdom you can change once you unlock and complete the respective questline.
Don’t fall for the newb trap of buying everything. 
Personally I get the gem sub but that is it for subs. I would prioritize armor as you said, then gem sub, then flash offers with gems as long as they’re cheaper than buying gems directly.
Play the game a while before spending anything beyond the armor. You will learn what you need and don’t need.
A lot of people recommend Deathknight Armor, but I honestly don’t rate it in terms of value anymore and think it needs an update.
The two main bonuses it gives over F2P armors are (1) bonus XP, and (2) bonus Gold/Souls, depending on whether or not you would otherwise have Dragon or Celestial Armor equipped.
(1) is largely irrelevant in the scheme of things, although I guess it might be cool to have a really high level at some point.
With regards to (2):
The amount of Souls you get from a regular battle when not using a Soul-farming team is negligible, and a bonus +50% to this probably isn’t going to make a huge difference at any point
If you are using a Soul-farming team, the modes in which it’s best to do so aren’t usually also the same places in which it’s best to earn Gold (high-level Explores, PvP).
Therefore, you can get away with using Celestial Armor when you’re specifically farming for Souls without affecting Gold tooo much, and use Dragon Armor the rest of the time when the difference in regular, incidental Souls will be negligible.
The main benefit of Deathknight Armor is therefore (imo) a Quality of Life one, in not having the navigate menus to switch between Armors, which can admittedly be cumbersome.
Tl;dr: Deathknight Armor is (imo) overvalued and needs an update.
Imo, the best things for a new player (/any) to spend money on would be:
A missing Event Weapon Flash Offer ($5 USD) for a Doomed Weapon that converts a colour to Doomskulls, e.g. Doomed Crossbow/Glaive
the $2 USD flash offers for Gems and Vault Keys
(Unfortunately) the Campaign Elite Pass in Week 10 of the Campaign if you have been able to complete all of the Tasks, as $10 USD for a Mythic (or similar) and other bits and pieces is pretty good value (just unfortunate because of the way it temporarily excludes F2P players)
Maybe the Sunday Dungeon Offer ($5 USD?), I haven’t really done the math on that one, though
Don’t fall for the obvious scam bait “sacred treasure” for $10 with your impulse purchases.
They love to present this to new players who then think it is an actual Mythic. Its not, its fodder for horde levels.
The best and only deals new players should take advantage of are the Sunday $5 diamond pack in dungeons and if you get a $5 Doomed weapon in a flash offer. The rest are overpriced and not worth 5% of what they charge.
I have to agree and thank the above, after some more reading and talking to guildmates I heartily concur
The only thing reasonably priced and worth buying are the campaign passes
But I wouldn’t do that either if I were you
Just join a decent guild and play, and you’ll get everything eventually if u keep on the game
I wouldn’t recommned buying it either, at least to a point when OP is 100% sure, he will do all the tasks.
So probably think of buying the pass once you reach 1000 camapaign stars (all elite pass rewards will be sent via ingame mail at this point).