What will happen to EB-5 investors already in the U.S. if USCIS denies pending I-485 petitions (AOS) during the lapse?

If your stay in the U.S. is pursuant to "adjustment pending" then a denial of I-485 will result in you being out of status, and, for most, unlawful presence would begin to accumulate. You should speak with your attorney to understand what this means for you specifically, but most would want to leave the U.S. within 179 days from the denial.

I am anticipating a LOT of litigation against USCIS. Against RCs? Might be a lot tougher. A lot of their offering documents likely had "the program could go away" as part of the accepted risk starting back in 2015 when we first started all the short-term extensions.

All good questions no one knows the answers to. But so far nothing is being denied, just on hold.

First, Congress will extend, and second, these folks will be able to adjust. In the highly unlikely event that they do not extend, I still do not think USCIS will issue notices to appear if the adjustments are denied. If the applicants maintained underlying status such as H-1B or F-1, there is no issue, and they simply continue. If the applicants did not maintain underlying status, we will have to find other visa options so they can stay and/or move forward with their green card applications. They would want to look at all options - EB-1, EB-2, EB-3, EB-4 and FB-1, FB-2A and other options such as asylum, cancellation of removal, and also benefits to immediate relatives. Also, some can qualify for DV lottery green cards. This all assumes Congress will not act but it will - it is just slow.

Under the current rules of prosecutorial discretion, CIS would not start removal proceedings. Instead, these people would be left with no status. I cannot see how litigation could help, since Congress is not required to extend the regional center program.