"1. Is it allowed for a single switch install with a 14-2 in and 14-2 out to twist the grounds together leaving one longer for connection to the ground screw?"

Not without some listed connector.

"Is it necessary to attach both ends of a 4 gang box to studs?"

No, as long as the box is solidly mounted.

"3. Who makes a quality non-metallic box at a reasonable price? "

Define reasonable.

Not without some listed connector.

So I have to pigtail?

Define reasonable.

How about at a fair price?

Use an Ideal Greenie.

The pros generally frown upon the blue Carlon boxes. I say pick a box & be done with it. It's not rocket science.

"Use an Ideal Greenie"

Or any other wire nut for that matter. Don't go out to buy green wire nuts for just one connection. ANY listed wire nut is suitable for a ground connection. Green wire nuts, on the other hand, are ONLY listed for ground connections. That's why I don't even buy them.

Not without some listed connector.

Not that I'd recommend it for most amateurs, but how about a mechanically secure, soldered and taped connection?

I'm enough of an old timer that, in my book, twisting and soldering beats any other splicing method. The twisting provides the mechanical strength. Solder reinforces the splice and seals out air.

The tape has to supply insulation value equivalent to that already on the wire.

"Not that I'd recommend it for most amateurs, but how about a mechanically secure, soldered and taped connection?"

While possible to still do, it is way more work than wire nuts and other much easier approved methods.