too many passengers are price sensitive and would go from LPL
Actually, this is a total myth. I travel regularly to Eastern Europe. In the last year I have used the Ryanair services from Manchester to Modlin (twice), Gdansk and Rzeszow. Previously I have also used the Manchester to Krakow route. In all cases, the fares from Manchester are very competitive - travel to Poland is really good value, and (if required) onward domestic flights within Poland seem absurdly cheap by UK standards. The notion that fares could be meaningfully cheaper from Liverpool (or Doncaster) is laughable. Just how cheap do they need to be? If there is any price saving at all (and that is doubtful) it would certainly be a fraction of the Manchester to Liverpool rail fare. Obviously, my examples here refer to the Polish routes from MAN/LPL, but I don't doubt that the principle would extend to a prospective service to Romania. Flying from LPL is not noticeably cheaper on these routes for advance bookings. Arrangements made at short notice may differ, of course, (MAN flights to Poland are invariably rammed) but I haven't had reason to check this.
If you think advance fares from MAN to Poland are priced uncompetitively versus LPL/DSA departures, check again. The only problem ex-MAN is that the routes are vastly under-served and you have to book early to secure availability. There is a pressing need for MAN to secure additional capacity to this part of Europe.