If I remove the two small screws on this plate/bracket under the driver's seat, so I can look for rust etc,
will anything bad happen, or do the bolts just hold this bit on?
Those 3 shiny bolts hold the torsion bar. Bad things will definitely happen if you remove them. There is a lot of spring in a torsion bar and there is a procedure for removing them. For the other couple of nuts leave them alone. I think the bracket is welded to the floor so it won't help.