Sooo many differing opinions out there. Would love to get closer to an answer.

I'm about to install 1600 sq. ft. of 5" beetle kill pine, tongue and groove, on a wood subfloor that also has radiant heat underneath. I've gotten a wide variety of recommendations on underlayment. Everything from - don't use anything - to Aquabar, to newspaper, to wax paper.

Kind of confusing. Want to make sure it's done right so I never have to pull or re-do.

Thank you!

I f you acclimate the flooring before you lay it, then keep the same conditions you will not need to put anything over the plywood, assuming ¾ inch.The American Plywood Association says that the perm rating for ¾ inch plywood is .1 perm, which prevents most water vapor from coming through the plywood.
Aqua bar, which is a heavy wax paper will help prevent a big spill from wetting the sub-floor if you have a small disaster. I would spread AquaBar just to make it more pleasant to work over.
With a smooth paper covering surface you can sweep up dust and cover dust that you can not sweep up when preparing the plywood. Lay the paper perpendicular to the flooring and overlap a couple of inches, then tack it down near the first rows.
Flip it over after it comes off the roll.

Depends on whats going on below the subfloor, if its open and ventilated to the outside then paper helps slow down any moisture coming from below. The NWFA guidelines still say to use paper and not newspaper or red rosin paper either.

Most flooring guys agree that 5" boards should be nailed AND glued. Wouldn't the use of glue make using paper pointless?

I have heard that some installers, whether a pro or not, slice through the paper with a bead of glue across the planks every so often and at the butt-joints.
Glue may be recommended but is not a requirement for a good floor. Pine is a softer wood and is more stable than most hardwoods, although plain sawn is not as stable as vertical grain.
A full-trowel glue down will not need other fasteners and will also provide a moisture barrier if you use the right product. It usually goes down slower and can become messy if not carefully done.