Where are the solutions to the problems?

I don't see them. Perhaps a reasoning LLM can generate them. Anyway, my interest is simply in learning techniques that are relevant to programming.

Where can I find solutions to the problems in the course?

We have been using empty interfaces to simulate F# discriminated unions for the last 5+ years. It works like a charm. The problem "not all cases are handled in the switch" occurs very rarely (once per 50k lines of code) and, usually, we fix it after the first run of a smoke test. So, it's not a problem at all.

I think,.net team haven't added discriminited unions yet because it can be effectively simulated with interfaces.