Well, actually, I have a harder time to believe that an experimental serum could give anyone limitless power, including resurrection and the possibility to stalemate Galactus.

I suppose that it is a metaphor about nuclear science in the wrong hands but I don't buy it.

On the contrary, Sentry/Void being an unstable version of DC's Spectre, for example, I have no problem to buy that.
It does not mean there is no Robert Reynolds. He would be the host/avatar of the Void, that's all.

If I comment some of the examples you gave me, for example :
- Both Norrin Radd & Michael Korvac had their powers thanks to Galactus.
- Owen Reece was powered by the Beyonders.
- Michael Pointer had the collective power of Earth's former mutants after Decimation.

I'll agree with you that, Scarlet Witch seems to be a mutant with unlimited reality warping power.
In my opinion, her power comes from Chthon, that's all.
http://www.marvunapp.com/Appendix2/chthonother.htm

Sentry/Void killed both Loki and Ares.
He is not very far from a character like Atum/Demogorge, for example.
http://www.marvunapp.com/Appendix/atum.htm