Re: The engines, albeit 50+ years old, are solid.
No, they aren't. OSC has blown up over half the engines they used to date. The Soviets canceled their moon shot program because they had too many problems with these rocket engines. That's the whole reason these rockets have been mothballed in Siberia for the last 55 years. Maybe the design in good and the manufacture was flawed. Maybe the design has a flaw we haven't recognized. I'll grant the most likely cause was a problem with the OSC refurb or the assembly of the rocket on the pad. But no matter how you slice it, using this particular engine was a BAD decision.
Yes, the root cause of the program is the US Shuttle program. It pretty much scuttled our own rocket engine manufacturing base for the last 30 years. Everybody is now relearning things that should already be well known.