yeah i already reduced the upcoming encounters down to only 1 (the final one) but sending reinforcements sounds a little too much like "i want you to survive this".
in addition, when a NPC party comes along i know what my players would do.... attack them. ;-)
tell you what i'll do, i'll keep the final encounter as it is, but i will include a trap in the room before, and also some cover possibilities. if the players are smart enough they can lure the enemies into the trap and/or into a very bad position for them. this should increase my players chances IF they play SMART.
the encounter is not so difficult by itself as well (LVL3, 700XP) with a controller, 2 soldiers and 2 skirmishers. the party is level 2.
they'll also get the usual action points for their last encounters and then that's it. for me it sounds more than doable.
if they survive they hopefully get the feeling that they really overcame some thing difficult.
if they die they'll have to play good characters from now on, including at least 1 cleric. ;-)