Ah ok, this sort of clarifies the issue, which is that you can't really run more engines at lower thrust, and if you deal with the tank pressure issues from S8 then I imagine it should be able to go down on 2 engines alone without catastrophic issues.
With respect to failure on ascent, I imagine in that case they just send up another starship and transfer in orbit (coming down has always been the hard part).
There could be a delicate dance somewhat similar to what Soviet lunar lander was planned to do at launch: start all engines, and if all is well, shut down those backups. Here with Raptors one could imagine launching two engines, but if one runs well - during the startup sequence - shut the other down mid-sequence.
But this is totally question to Raptor creators. Might be not really feasible or even meaningful.
With respect to failure on ascent, I imagine in that case they just send up another starship and transfer in orbit (coming down has always been the hard part).