...I'm skeptical that the difference in the weight of the 1mm stainless steel valves will cause them to float at 5500 RPM, even if the springs were stock.
I think you're making an assumption there. In my experience, assumptions are defined as things that prove to be false after costing you large amounts of time, money, and tears. Until you measure, calculate, or verify every one and find the one that was wrong.
Sorry to hear your engine rebuild went west like this. With all this experience, the next one will probably take you a couple weeks and run like a top