What if I let my Eb-5 conditional permanent residency expire and apply for a green card though marriage in the U.S.?

Issues are more complicated than just letting it expire. What is your legal entry then? You will have to formally abandon it, not just let it expire, and this must be done out of the U.S.

I am pleased you asked. In the past, I would have said you need to file the marriage based petition and then surrender your conditional green card, but I am pleased and proud to say we just assisted as local counsel on a case filed by another lawyer that was approved in precisely these circumstances. So I am pleased to advise this is a viable option and more than that, I am proud to say we have actually succeeded in adjusting such a person so there appears to be another option besides having them surrender the old card and apply for a new one.

It is not an easy process. You likely will have to renounce and re-process. It might be better to just file the I-829. If you are worried about delays, consider mandamus. I have had success with investors waiting as little as 5 months.

You should be able to do this; speak to a lawyer as there is a special way to file for adjustment when already a conditional permanent resident.

You cannot adjust status unless you are in a valid nonimmigrant status. The cleanest solution is to file I-407 abroad, but you should not be coming back on a nonimmigrant visa with the intent to adjust status in the U.S. Your situation is very complex and you should discuss it with an experienced immigration attorney.