Darkveil Agent should be used singly or to prolong Nightfall. You want one alive on the board at all times to keep your valuable units alive and continue buffing Shelterwing Rider.
Use Nocturnal Observer to dig for cards you need, pitching extraneous units with novel battle skills--you still gain utility from these units when Honor the Fallen is played. When strategically relevant, deliberately pitch high-value units just prior to using Honor the Fallen.
When Bisma dies, you get the dual benefit of transferring her Battle Skills to another unit on board, and granting the same set to a second unit using Honor the Fallen.
That being said, I think a fourth Darkveil Agent is your first replacement for Kothon. After that, probably Spirit Guide or Aerialist Trainer. Wardwielder seems sketchy with only Protect for face aegis. Possibly a fourth Vanquish wouldn't be bad. As for Nostrix, Eilyn, Clan Mother is probably about as good, though that isn't exactly a budget replacement. Mistveil Drake isn't too bad, and pairs nicely with Wardwielder if you run it.