OK Ty, it's taken me some time here to assemble my thoughts...
I'd stick to the plan and go with your second option above. The LV12R will be perfectly fine in your room and for your needs. The F15HP will dig a little deeper in-room due to the shallower roll-off of a sealed design, but has only marginal extra output (1.5dB) at 20Hz. Also, if your thinking ~$1100 for subwoofage, you'd be far better off adding a second LV12R at some future stage. (Not so much for extra output, but for a smoother bass response over the listening area.)
When ordering surrounds, there's a pair of HTM-200 SE's in
Ascend's B-Stock that might be worthwhile considering. Will you also need pedestal stands for the 340's? There's a pair in B-Stock as well.
All that (mains, centre, surrounds, sub, stands) loaded into Ascend's shopping cart comes to $1780 shipped, which keeps faith with the freshly negotiated budget. I suspect this will stand you in good stead for when it comes time to broach the subject of a big new plasma again.
Don't worry too much about the wait... the 340's will keep a
on your face until the sub arrives!