I have a M&P 15 in .300 BlackOut, RealTree APG, 5R 16" barrel, all factory. I don't think the hand guard is two pieces. I'd like to get the most basic adaptor that would allow me to use a Harris Bipod that attaches through a swivel sling stud. Any sources, and how easy is it to remove the hand guard if necessary?

You can get a handguard removal tool from almost any place or you can use the buddy system.

Wheeler Engineering Delta Series Handguard Removal Tool AR-15 Steel

You can also get a Harris bipod adapter from several places.

Harris Bipod Adapter Stud Swivel #5 AR-15 Black

Unless the handguard is aftermarket, it is two piece.

If the picture I found of your rifle was correct, the best way (imo) to mount a bipod would be a to get a new hand guard that has rails or mlok/keymod that you can attach a small section of rail to. Then buy an adapter to mount the bipod to the rails. Harris makes these if you go with their bipod. The above mod to attach a swivel mount would work but wouldn't be the most secure or accurate way to attach it.

I've used the #5 on an A2 rifle forend, but was never pleased with the degradation in accuracy. Found bipods worked much better on an AR15 with a free floating forend.