Any question answered...

Flip the tri square the other way round so the wood bit is pointing away from the handles, you'll be able to determine if the plate is flat or not just using a straight edge.

If you want to find if the foot plate is perpendicular to the cutting tool maybe clamp the router in your work mate and then use a level.

More importantly, are the slight variations in cutting angle negatively affecting the work piece? Nothing will ever be perfect, as long as you're with in tolerance it's all good. Trying to achieve perfection with everything will just drive you mad.

The foot plate is warped, as if you took a tray, held it at either side, then rotated your hands one toward and the other away from you.

Now the question is- is the warp within tolerance/accepted, and I'm just being silly?

I'll cut the rebates in this door lining and see.