How TF does gear, boons and purifying work ?

It seems so convoluted, I don't get how gear works. I'm currently running in all blue gear, I see a white drop with boons that I like but don't want to wear right now so I reserve it but upon extraction it loses a boon.

Do items only keep the second boon when you equip it immediately?

If I equip an item with 2 boons, then swap it out at the start of the new run, will it lose it's second boon in the armory?

Do items keep their rarity when you reserve/equip them, or you have to upgrade them at the blacksmith after a run?

Which boons can stack? I have an item that gives 2 rage on sprint attack, I picked up a end-level boon that does the same thing, thinking I'd get 4 rage per attack but no, still 2.

So many questions..