This is why I am highly sceptical it can be part of a commercial supersonic passenger jet: how do you get from subsonic -> supersonic without also tacking on some kind of conventional jet engine?

Concorde was commercially viable at Mach 2.2 in supercruise (although there's a common misconception that it was not).

However, its overheads were very high, and its applicability was severely limited by fears around the sonic boom (most particularly in the US, which banned supersonic flight overland, possibly largely because they wanted to kill off foreign competition).