What happens when camera overexposes images shot on white seamless?

You are shooting RAW (right?), so the issues you are having are probably more to do with your raw converter than anything else.

WB is not an issue with raw.  Personally I set my camera to Daylight and leave it there.  I make any necessary adjustments in the raw converter.

Your Minolta meter, I presume, is a flash meter, so the meter is reading the incident light and not the reflected light.  So, assuming your flash meter is working properly, the exposure should be correct, or at least well within the ballpark.

Maybe you need additional light on the background.

I would try bracketing a few shots to see what works best.  But you should be able to get everything adjusted to where you want it.

I also do a test frame with a reference card in it. The one I have has black, white and 18% grey patches.  I also have a Macbeth color chart and an X-Rite passport.

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The greatest of mankind's criminals are those who delude themselves into thinking they have done 'the right thing.'
- Rayna Butler