I wonder, if I were to replace whatever fan in an SFX-L unit, whether a retail slim 120mm fan can fit inside snugly without scraping the fan frame off to make the fan a tiny bit smaller.
Yes, any 120x120x15 fan should fit just fine. The outer measurement of all SFX-L cases so far is 125x130mm, so when subtracting the thickness of the case material there'll still be at least a couple of mm of space around the fan frame.
Probably. Or people could select a fan with inadequate air flow/static pressure.
Most likely, yes. The potential for disaster is very high.