In a way, I'm still finding for that holy grail too.

Don't get me wrong though, I too am indeed looking for a full PC housing (transparent or opaque) switch that's cheap enough to just harvest its housing. But, as horns mentioned, most such switches still cost more than RM2/each, still not exactly worthwhile to just harvest for housing.

It makes a difference if there's no other mods done, but when you compare "foam between plate and PCB" and "foam between PCB and case", the difference with and without foam between the plate and PCB is larger. Of course, as with all mods with keyboards, there's no single recipe, it's always case per case basis (pun unintended), some cases produce more ping and thus requires foam to tame them, some cases are well built and fit together so well there's very little hollow sound hence the marginal difference.

T12 style iron/tips are "better" then typical 900M style tips for 1 reason. The heatercore is already built into the tips instead of the soldering "gun" attachment, faster heat up, more consistent heat. For a hobbyist, the standard 900M or T18 tips are good enough, I would recommend splurging on original Hakko tips for better results.

Typically M2 4mm to 6mm or so, you're supposed to only screw the PCB into the case, the plate is held by the switches which in normal soldered builds, is not an issue at all. If you really want to lock down the plate along, add 5mm to the screw size, and also you'll need to add a 3.5mm "washer" in between the plate and PCB.

T12 is different from T18. T18 is similar to 900M. T12 has the heatercore in the tip, hence the price per tip also costs more. T18/900M has the heatercore in the handle.