Boosters are stripped from enhancements when they're moved to the market window (deleted, not moved to your salvage storage), and the enhancement is restored to its crafted level. TO/DO/SO/HOs can't be boosted, nor can any attuned enhancement (which includes Winter, Universal Damage and ATO).
Boosters don't alter the level of the enhancement. A 50+3 IO is a level 50 IO with improved stats, not a level 53 IO.
Also, boosted IOs never "don't work at all" due to exemplaring. If it's a set IO, you can lose set bonuses when you exemplar more than 5 levels below the crafted level, but not the effects of the IO itself.
TO/DO/SO/HOs can't be attuned, IO recipes cap at 50 and crafted IOs don't drop.
Booster don't drop. Merit vendor, super packs or market only.
Well, you were right that attunement caps at 50.