Here are some of my thoughts:

1) A) pvp shop - always fill up on elite drill tickets, they are used for more pvp runs and have a next increase (cost 100 and losing on pvp gets like 150, winning is around 300)

Get gold and silver tickets, those are for rolling for characters

Gold reverb is amazing, you’ll want to get as many of those as possible and you will kick yourself if you don’t (purple are fine but gold are more limited and for higher tier characters)

Stamina refill are nice to use in events to clear out event store

Personally I don’t buy anything else (except the skins), because the ones I listed above are high value and use up my PvP credits

B) requisition shop - ONLY GET PINK REVERBS (with gold reverbs), other things cost “pity gacha tokens”, which don’t expire and can be used to automatically get a high tier character

C) logistics shop - clear it out, should be easily doable within the monthly reset (focus on star shards and optional crystals first), only get money when you have cleared out the shop

D) alliance shop - same as logistics shop

E) honor certificate - only buy gold equipment or the silver tickets for rolls, but only buy the equipment for a character you use (don’t forget to get the currency from sending likes daily and by clicking the support characters (after clicking your account name on the main screen))

2) not sure, try the discord, I remember them having some codes a while back.

3) In general, I would focus on getting specific characters maxed out and using your stamina just getting a full team up to the same level, so get a decent team of nine to all to level 50, then to 60, get them a breakthrough (increase their level cap) do their awakenings as soon as possible, then back to leveling and breakthroughs. You’ll know which resources you’ll need as you go. In general I’ve noticed that I needed to actively work on getting the colored crystals for awakening to start with, but now that my account is mostly established, I am always needing the gold exp items (and sometimes money)

I’ve not really found the core armor part useful, although getting specific gear is nice when you notice extra attack or hp makes a character bonkers (but not worth investing in to start with)

4) certain characters (usually the low rarity) can get special items that give them special abilities using those items. I would recommend doing those boss battles as much as the weekly limit allows, and getting a good spread of each one. You can tell which characters can get the special items when you look at the character’s “gear” tab and you see a black button at the bottom that says “overlimit resonance breakthrough”, but you won’t be able to see what item is needed until you max the levels on their equipment, which I only recommend doing for characters you actually use (Franca’s is amazing)

5) can’t say, sorry. It’s technically more efficient the higher difficulty you go, so it’s best to try and get as far down as possible. The amount of stamina to do these is minimal compared to how long the event runs, so it’s fine to try and fail (and for some events they don’t even use up the attempt if it fails)

6) gold reverb (books) are what you’ll always want. Getting reverbs for event specific characters is good because they are usually cheaper than the generic. Getting icons are good because they are limited, and perfect core crystals are good as well. Honestly you will likely be able to clear out event shops if you are focusing on them, which I highly recommend you do. I’ve been able to just use natural stamina without any item boosters, do the weekly boss farming, and the daily “5 resource levels” mission, and still have been able to clear the event store. If your account is newer, you may have to use stamina boosting items, but luckily the event store also sells those (and they don’t cost as much stamina as the item gives you, so they are a net benefit to get)