Installing mobo into case

Well, if the screws are popping out, they aren't the right screws. And the motherboard should lay flat and easily into the "cradle" of the screws and standoffs, there shouldn't be ANY pressure involved at all. If the motherboard is having to be flexed to make it fit securely, there's something badly wrong in the mounting hardware.

All the screws and standoffs should raise the same distance from the side of the case - that is, if you were to lay a level across any of the points of contact, it would read level. Some boards use only screw mounts, some have also plastic standoffs.

To answer your question, no, all 9 holes don't need to have screws in them. Generally, the two holes nearest the keyboard and power supply are the most important, the others are needed to make sure that when you push/pull items like adapters and memory into the board, the board doesn't wiggle or move or flex.

Watch out particularly for points of contact that don't match up with holes in the motherboard. You can't let those touch the motherboard - remove them or cover with thin cardboard and tape.

Remember, you have both "fine" and "coarse" screws that will seem to fit into the mounting points. If they are popping out, it's likely due to you using fine screws when the points need coarse.

But you really need to find out why the board has pressure on it when placed into position - there's something wrong there.

six standoffs is fine just spread them around, muck more critical are the points mongox made