Any advice on knowing how far to turn during the
first process so I don't have the SS Niles Stopper tenon poking
through? Now, what else do you think I'd be using!?! : )

Ruth,

If, when turning the inside, you just make the piece round, you will not
see any opening in the middle. The more you cut past this point, the
larger the open inside becomes.

Ideally you should face two adjacent sides of the wood on a jointer to
make sure they are dead flat and square to each other. Then machine the
opposite sides on a thickness planer. This is the ideal situation but
you might get away with cutting them square on a table saw with a good
blade.

And someone mentioned putting a spindle inside the opening. I haven't
tried this but there is no reason it shouldn't work. Turn the inside and
reverse the pieces but don't glue them together. Use double sided tape
or clamps but fasten them in what will be their final position. Then
drill a hole through the end to accommodate the spindle. Then glue the
whole thing together. Might be fun to try.

Harry