John, you have asked some hard questions which I will try to answer as best that I can.

The issue is: can it be measured?
Yes, alpha-synuclein levels can be measured in biological fluids (CSF, plasma, etc) - I would do this using an immunoassay. An immunoassay could also be used to measure intra-cellular alpha-synuclein levels in whatever kind of cells after those cells have been lysed, which means releasing their contents.

And, better still, is there a DIY way of measuring it and its misfolding?
The short answer to these questions is "No". Immunoassays and ways of measuring alpha-synuclein misfolding both need access to expensive instruments which are not generally available DIY.

For instance, aggregations of alpha-synuclein, Lewy bodies, can be seen in a microscope.
Lewy bodies can be seen in a microscope. However, that does not show that they contain alpha-synuclein - it needs immunocytochemistry, which I mentioned in one of my contributions above in this discussion.

I hope that you find my answers to your questions helpful.