Basically what the title says, if I purchase a gold membership will my lvl 50 character receive all the benefits associated with leveling up? Also what will stay and what will drop when/if I let the membership lapse? (Specifically that free character slot, but also other things.)
You'll get and keep almost everything except the EC cap (which becomes permanent if you buy it... well worth it in my opinion). The only benefits you won't get retroactively are the free Respec tokens. The extra character slot should stay active so long as it's occupied by a character... it'll disappear if you leave it empty when you revert.
also, if your max level, you wont get y9our increased inventory and bank spaces, since those are granted as you level up.
This is incorrect. By becoming 'Gold' all your current characters will gain all the inventory slots (e.g. Personal Inventory, Bank, Shared Account, BO Slots) that are due to your rank retroactive. And these gains are permanent, and remain if you revert back to 'Silver'...
Only non-inventory gains (e.g. respec tokens) are non-retroactive.