Concerning your questions:
- Yes - it will. As soon as the FP3+ is supported at all, you will have a common LineageOS build for both devices since the differences concern only a few supporting firmware files.
- I am working on 17.1 right now and hope to be done by Christmas. As to why the difference in implementation - the FP2 maintainer and I are working on these devices independently and I was late to join the party.