Interesting that this was brought up very recently, perhaps due to the recent news of Chinese researchers which used D-WAVE annealing quantum computers to optimize an approach to crack RSA.
However, the idea with BitVM is that any cryptosystem could be emulated. The Bitcoin developers would have some time more to decide which algorithm is the best one, and then could opt for a soft fork in the late 2020s or even in the 2030s, when the algorithms have been tested extensively. Meanwhile those who already want to secure their Bitcoins with a "candidate" post-quantum algorithm (or any "more advanced" cryptographic algorithm) could do it.
Yes, this was already known to me. I'm however not sure how much must be posted on-chain in the case a spend is contested.
Even if ECDSA will be broken, then still, there are scripts you can do, which could be safely used,
Interesting

Will try to investigate about this.
It seems also to be a fork proposal and not based on BitVM. I've still not found anything regarding the question (if BitVM can/should be used for this purpose).