In the general election the “feasible” candidate is always the Democratic nominee, so you should never have any argument about it at that stage. Meanwhile in the primary people try to use that sort of “feasibility” / electability argument against farther left Dems, but it is total nonsense and can be completely ignored at that stage.
Yes I definitely assume that. Maybe not every single person since who knows what goes on in people’s heads, but generally we should expect the voters for the two left of center parties to prefer the other left of center one to the right wing one. Particularly since presumably if there was a single party representing those voters it would probably be somewhere in between them ideology-wise.