Thanks guys for the input.
RSI: I agree, that it is low water flow, low air flow, or a combo of the two. I am swapping the pump out to the OWB as soon as I can find some pex-al-pex to NPT compression fittings. I think this is most likely the problem, as the furnace itself will keep heat in the house. I will double check the ducts for dampers, but to be honest, I can't remember ever seeing ANY in this system (wish there were though, as some rooms get much warmer than others). I will see if I can scare up a smaller adjustable pulley: one with the largest diameter basically the same as the current one's smallest diameter (where it is set now). That way, I can play with the adjustment. If I am still having issues after the pump swap to the OWB & playing around with the blower pulley ratios, I will lay out all the connections/fittings/lengths/etc in a drawing.
Scott: The modine may not be for water (who knows with what the PO had done), but it isn't part of the current iteration, as I physically disconnected it last season while trying to diagnose my heat issue (right now, all I have on the system is OWB->PEX RUN -> PUMP -> COPPER - > EXCHANGER -> COPPER -> PEX RUN TO OWB & some fittings/ball valves/y-strainer/air vent). I wanted to get my bearings on things before I go and muddy the waters further. Also, could you let me know if you have 1" compression pex-al-pex to 1" male NPT fittings available? I am going to do some more calling around today, but it has been odd that these aren't readily available here in North Central MN (I will be calling down to the cities today: an hour each way drive, but ISWIS).
Ijon: Good idea on a site glass. Will take a look around & see what I can find (even a flow indicator from grainger or mcmaster would work). I have used them before on some processing equipment, but I seem to remember that getting stuff which dealt with higher temps (200 F or so) were stupid expensive for what they were. That was a few years ago though, and the mind isn't what it used to be).
Again, thanks everyone for the insight. It is greatly appreciated.