A very unique HVAC situation - please bear with me. The living room in the house I just bought (built in 1950) has a walnut floor floating over a concrete slab (no basement). Air return is between the floor joists between the walnut and concrete, then traveling under some heavy stonework on its way back to a crude wood and concrete channel under the furnace. Air supply to the living room comes out in similar concrete-bottom and wood-sided channels below the main concrete slab level. Both supply and return vents have a strong wet basement smell, and I can't access their entire length due to the heavy stonework between the living room and the furnace, and the lack of any basement or crawlspace.

My current plan is to abandon the air return from this room and enlarge a separate air return closer to the furnace. I have thought of abandoning the supplies as well, which would rely on the built-in (and vented) gas space heater in this room, and eliminate central AC from this room (not a huge problem - the house stays pretty cool). But I could potentially salvage the supplies, running flex duct through the current supply channels from the vents back to the utility room with the furnace (we can't install normal sheet ducts under the stonework, but I think we could fish flex duct through the current ductwork).

Online and from local HVAC pros, I'm running into very different opinions on flex duct in terms of its air resistance. Some think it leads to enormous air resistance no matter what, and I shouldn't bother with it. Others think it's basically fine if stretched tight, and the negative buzz is mostly due to some very poor installation jobs that show up. If flex duct is my main alternative to keep forced air in this room (aside from much more expensive construction to install new metal ducts through the kitchen), do you think I should consider it? I'm looking at running 7 inch diameter flex duct 18 feet from the furnace to one vent, and another 7 inch flex duct 23 feet to the other vent. THANKS for any help you can offer!