What do you mean "how tight are they if you go over the lease mileage". The lease mileage penalty should be on the lease paperwork you signed.
Tesla does not operate like a traditional car dealer so they are not going to "forgive lease mileage if you buy a new car" (note that in all cases that happens, the traditional dealer is making enough money off you on the new car to do that, so there is never any freebies).
You will pay the lease overage charges, whatever they are in your contract.
The answer to the other question is also in your lease contract paperwork but the answer is currently "no you cant sell it, you cant buy it you can do nothing but give it back to Tesla".