It's playable (I recall the English GM James Plaskett playing it a few times) but tends to lead to a standard IQP position where White is a tempo down compared to the normal case.

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As you found it, no way exists to "force" a Korchnoi gambit position as White after c5 (the "open" main line)...and Bb5 is often the better move for White. Another point is Black or White often looses a tempo.
After 1.e4 e6 2.d4 d5 3.Nd2 c5 4.Ngf3  Black need an useful move:
A) 4...Nf6 5.e5 Nfd7 run to Korchnoi's gambit where White as space advantage
B) 4..cxd4 5.exd5 Qxd5 (5...exd5 6.Nxd4 and White has already his pieces on good squares) 6.Bc4 is the main line .. and you 'll note White castles quickly. The pawn less (d4) is quickly recovered (usually Nb3-Nxd4)
C) 4..Qb6 5.c3 Nc6 (5...Nf6 6.e5 see A) 6.exd5! (and not 6.e5?! cxd4!  or Bd7 with transposition to Nun's gambit) exd5 7.Bd3 (better to castle and threaten Re1 early) cxd4 (7...c4 Bc2 and an easy game for White with b3 in some moves) 8.cxd4 Nxd4 9.oo! a gambit will Black has many problems (king safety, d5 isolated pawn)
D) 4...Nc6 5.c3 if you want to transpose to C but 5.Bb5 a6! 6.Bxc6+ bxc6 7.c3!? is interesting too, with play for White on dark squares. Rohde-Gulko
or 5.exd5 leading to lines with the isolated pawn
E) 4...a6 (I noticed Korchnoi,Gurevich like to play this move which is start of system where Black plays a6-b5-c4-d5 pawn structure) 5.dxc5! Bxc5 and now White has two plans

E1) 6. Nb3 Bb6 7.exd5 exd5 8.Bg5 Nf6 and now 9.Qe2+ Socko-Kiriakov,Saint Vincent 2005 with a sharp game. In the position with isolated pawn on d5, it's important not to give Black active play (for example: Bb6 and Nf6-Ne4) and sometimes Bb5 is used to entice g8 knight moving on e7 instead of good f6 square)

E2) 6.Bd3 Nc6 7.oo (7.c3 Nf6 8.Qe2 Qc7 9.oo Popovic-Kiriakov,Internet playchess.com 2006 and 10.Nb3 Litwak-Kiriakov,Internet playchess.com 2004 ) Nf6 8.Qe2 Qc7 9.h3 Renet-Gallagher,Clichy 1989 or 8...Nb4 9.e5 Nd7 10.Nb3 and small advantage to White