If you’re using plywood you have nothing to worry about – ply won’t move! So either of your solutions will work. Option 1 (the imposed/inset kempas) would be simplest.

The panel-in-groove solution is designed to get around wood movement issues because you don’t glue the panel – it floats in the grooves and can move as it likes. If you’re using solid wood (as opposed to plywood) the only tricky bit is sizing the panel – you want it of a size that if it shrinks or expands a little, the grooves can accomodate that.

1. Glue a piece of plywood in the middle (would that give me any problems?) and then on top of that (and on the underside), glue a very thin resawed piece of kempas. Maybe chamfer the edges down to meet the surface. I might have to glue another layer of wood inside the mitered edges first, so that my very thin inlay doesn’t get covered up.
2. If gluing plywood to the edges is still dangerous in terms of wood movement, I could try to route a groove in the inside edges (I do have a mini router plane) and have the aforementioned piece of plywood fit into there. I suppose if I do that, as you’ve suggested, there could potentially be a bit of looseness in cool/dry climates? (I’m in hot humid here.)

If gluing the plywood would work, I’d rather do that since routing a groove would be yet another step in this process. Although it’s all in the name of getting experience so I shouldn’t complain.

Let me know what you think!
Eric

But there are two problems, I think. First, as you’ve discovered, sizing the panel to fit is tricky. Second, if you simply glue the panel in there you’re risking issues with wood movement: if your panel is as dry as it’s ever going to be, increased humidity will expand it and either bow the panel, or force your mitred corners apart. If it’s at greater moisture content than (say) the air-conditioned interior of your home, the panel may well shrink, splitting at the weakest point, or pulling away from the edges leaving unsightly gaps!

The age-old solution is the floating panel (as in cupboard doors). Cut a groove in the frame pieces, size the panel to fit; the groove gives a bit of ‘wriggle room’ if you get the sizing a little wrong.

But the way you designed the frame gives you another option. Split the sapwood pieces in two along their length. Fix the top-most pieces. Then fit the panel. Then fix the panel by using the bottom lengths of sapwood to trap the panel. I’d guess this is how you’d have to do it if you went with your mirror idea (or think of glass windows in a multi-panel door).

I’m not much further along the learning curve than you when it comes to handtools (or woodworking in general, actually). But I think these sorts of issues are what make handtool working trickier. The normites would just run the frames through their router tables for the groove; a few quick cuts on the table saw to size the panel, and voila!… a perfect fit.

Hope this rambling makes some kind of sense!