You make a good point there at the end...Optically, it will most likely work. Mechanically, I don't know how well the focus ring on it will line up with the gear in the housing. That said, I don't know about your level of experience in underwater photography, but a manual fisheye lens is not the right way to get started. Fisheyes are very challenging to properly frame, and for many shots they don't work at all - and using one with manual focus and fixed aperture adds another layer of complexity. While they're quite forgiving of focusing mistakes, they still need to be stopped down quite a bit to achieve significant depth of field, and this means relying on strobes extensively - and with a manual lens, you can't change the aperture underwater to adjust for changing conditions.
Forget about the 12mm f/2.0 - rectilinear lenses require much more precise focusing; autofocus is a must.
If you just want to dip your toes in it, Meikon housings for NEX5N are just $110 on Aliexpress - there's a version for 18-55mm lens and another for 16mm; both accept wet lenses.
If you go the A6300/A6500 route, the 16-50mm lens can be used in the bundled flat port, but it doesn't take wet lens attachments. If you want to use those, you'll have to buy the short macro port. Alternatively, you can use it in the 6" dome. I have shot it (the 16-50mm) in the old fixed-port housing with and without wet dome attachment, and on one dive, for a test, in the new housing with 6" dome. Unfortunately I got a regulator leak on that dive and had to abort, so I only go a couple shots - here's one for reference:
View attachment 486177
As far as size and weight go, the housing's contribution is relatively minor, and it helps that A6xxx cameras are quite small to begin with. After packing a tray, two arms, two float arms, six clamps, two strobes, two large lights, one small light, twenty 18650 batteries, sixteen AA batteries, two chargers, spare camera battery + charger, vacuum test system, spare o-rings, lanyards, half a dozen lenses, several ports, regulator, dive computer, BCD, wetsuit, fins, clothes, shoes, toiletries, laptop + charger, power bank and probably a dozen more things that I'm forgetting now, a camera housing is a fairly small addition.
And lots of food for thought overall. Thanks.