If it already worked then you already have the cpu dummy  chip on the otherside of the board. For vs smb, it needs either the dummy chip on the left side cpu slot or another game on the left side. Since yours worked before, you must already have it.

Did it work before? And now it doesnt work? Do other games work in the cab? If it vs smb and its in a single monitor cabinet it needs to have the cpu dummy chip or  jumper wire to work.

You either need to push harder or get a different board. If you put a regular cpu or ppu in the sockets do they just fall out? If they do you need a new main board. If they click in and stay, then you are just not pushing hard enough. With my castlevania card i took the cpu and ppu off so i could really push down. Just when you think it is going to break it might be in .
Hope that helps

As long as you dont run the game 24/7 you dont need the heatsink. Its nice to have but not necessary. I have a few without them, bought that way, and have never had a problem

Not likely to be a static problem, most of the older boards and chips are not static sensative. They should survive an em pulse.