Some manufacturers recommend overlapping the 15 lb roof paper while others say to butt together. Which is the best way. I also am guessing it is stapled to subfloor. Also would it be a waste of time or a good idea to use tongue and groove glue on a stapled down 7" wide engineered plank floor.

Phil

The reason for the paper is to retard moisture travel up through the sub-floor.
Butting the edges will be sufficient, because a small amount travels through the paper anyway, so with a tiny "leak" every three feet the moisture coming up will be negligible.
The paper may be easier to put in place with overlaps so there will be no need to cut the paper to fit the room. If you will be overlapping the edges you should install the paper perpendicular to the hardwood flooring so there will not be a thicker layer where the t&g comes together between rows. The t&g on the butt ends are milled with more tolerance so the ends will go together even if there is an overlap right where the ends come together.
If the manufacturer recommends glue along with stapling go with it.
It will take more than twice the time to lay the floor with a full trowel glue. The glue can be messy. You can spot glue every so often, but there will not be a moisture barrier unless you cut it out to the sub-floor every time you want to use a bit of glue from a caulking gun

Check out Aquabar B as a moisture retarder. It's really nice to work with.

I'm installing 3/8'' Maple Prefinished Solid flooring to plywood sub-floor with Powernail model 50p flew foot. The 18 gage Powercleat is splitting the tongue on the wood. Why is this happening?

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Do you have the correct spacer plate for your flooring?