1. The LFTR fans think an inherently low breeding ratio is a good thing and always advertise "non-proliferating" as a feature.
This of course makes the design very delicate (or unreliable if you like) in terms of neutron balance.
It also makes the LFTR dependent on some external source of fissile material.
2. and 3. are also true.
4. is what worries me most. The cracks and corrosion are not a trivial problem and have never been shown to be solved.
The presence of dissolved fission fragments means you are running a very complex chemistry experiment with your pipes and pumps.
Most of our experience is with solid fuel where the fission fragments are well contained. It's going from one extreme (well controlled) to the other (worse possible case for fission fragment attack).
Your 20 year time frame seems like a minimum to me. We need to add to that the time required to round up funding.