The felt comes in rolls of any width you might want and also in various thicknesses. It is durable and mildew proof and serves as a insulator to some small degree. Worth going for for those reasons alone but the "no squeek" function is justification all by itself. Carpet does age and then it crumbles and unravels. Iffy, at gest, and especially so if you are intending on using "old stuff".
The bays are different. You car R&R that stuff down the road without any serious hasstle. The floor....well, you will be building the entire interior on top of that and it is a serious commitment point.
I am not sure that the 16 inch center cuts was a good idea. The more joints you have the more screw up you MIGHT have. I have only seem the floors installed with the 4 X 8 sheets laid across the bus and tongue and groove edges and the joints also glued.
You might consider holding the felt in place with a spray of contact cement. Don't bother with the "wait till it is set before contacting" procedure. Just give both surfaces a good squirt and slip them into place. if it lifts a slight pressure will re attach it. You can then concentrate on getting your sheeting down and not worry with the felt staying put while you jockey stuff around on it.
All that stuff you said about treating the underside before installing the material is good. I would go with the epoxy resin, if it were mine. Blocks moisture, as you pointed out. You are still going to install spray foam under the floor....right? That stuff will seal your floor to moisture all by itself. Might save a step and spend the material cost somewhere else. Even though the epoxy under the foam would be redundant, I think I would do both, but then I am so very RICH that money means absolutely nothing.
Keep us all posted,