i've laid 12mm plywood over chipboard for wetroom floor:

1. do i need to fill the expansion gaps between boards with anything before applying adhesive for tanking membrane?

2. i have mapei primer, what ratio shall i use for priming the floor/plasterboard walls?

3. if i decide to get non-shower wall skimmed should i do so before bringing tanking membrane from floor up 75mm onto bottom of wall?

4. whats the best method of cutting out floor area for former? circular saw (with the guard removed) or floorboard saw?

use flexi and tape for the joints

and dont remove the gaurd from your saw under any circumstances

you can hold it back and plunge cut, dont stand behind it in case it grabs and jumps back at you

Use a circular saw, you can set the blade depth to the same depth as the timber, that will ensure you can cut only the floor and not any hidden pipe work or cables.

As Mike said i wouldnt take the guard off.