plasterboard fixing foam

Some - but not as much as regular.

Yup - basically squirt it on to your required thickness and a bit. Slap the board in place and push it to where you want it. Wait for it to set.

Don't know... it will stick well, but I don't know how well the traditional adhesive will stick.

I've used that.

About 2:1 after the initial expansion from the nozzel. It is *very* sticky stuff and bonds well, quite a firm foam when solid, more than regular foam.

It has little "grab" though so I suspect you need to support the board somehow.

It will set to any thickness - I used to to glue celotex in to a bay ceiling where cutting a firm fit was difficult due to crossing timbers.

It will stick well, but I think the regular adhesive would too.

Cheers

Tim