Yes
yes it can
The 1975 regular Beetle windshield will fit the 1971 Super Beetle.
Yes I have a 64 and repairing the quarters with 65 skins. They are identical.
No, the M3 has different quarter panels, which are wider then the 330. If you are a body repair specialist, you could get them to fit with some creative work.
Assuming you mean 1973 Ventura, some parts will (door skins and quarter panels, for example)
No.
a 912 Porsche engine will fit in a beetle
The elky hoods are the same. The monte won't fit without changing everything from the quarter panels forward. Not sure about the malibu.
Yes, since that's a tautology. Unless you mean "would additional seats fit in a vehicle without taking the original seats out first", in which case it would be a pretty tight fit, but you might be able to get them in the back. There isn't much room in the "trunk" (which in a '74 beetle is actually in the front), and I doubt you could get them in there.
1965 to 1979 is the same.
That probably isn't a good idea for multiple reasons.If your new panels weren't the exact same as your old panels they would never fit.Even if they did fit over the old panels, when moisture got in between the old and new panels it would create problems.You wouldn't be able to fit new trim over the old trim.
It should.. I have a 79 1600cc in my 65 beetle.