Hi,
I want to make a countertop for a kitchen island that would create a
separation between the kitchen and the living room. In a building
materials salvage store, I bought:

A solid cherry panel (3/4" thick)
Mahogany boards (1.5" thick)

My plan is to glue another panel to the underside of the cherry one to
make it thicker and make a ~ 1.5" mahogany border around the
perimiter.

The project is complicated by the fact that the island is an irregular
hexagone.

As I have very little experience with woodworking, I would appreciate
any constructive advice, especially:

1. What should I glue to the board's underside - MDF or plywood? What
gue should I use?
2. How do I attach the border - just glue, glue + tongue-and-groove,
biscuits?
How do I join the ends of the border segments - miter or rabbet
with dowels, or? If the segments are just glued together, is there a
risk that they could separate due to the board's shrinkage and
swelling (even if it is glued to MDF)? What kind of glue?
3. Routing the outer edge of the border - should I glue the border and
then rout it, or rout the segments before gluing on a router table?

Thanks,
iouri