"The pieces are all jointed and biscuited."
You need only to edge glue the pieces while glueing the biskits. No need for additional reinforcment.
Welcome to woodworking and good for you on all accounts,teaching yourself,tackiling a project then seeing it through and getting rid of the POC.
Not to flame G.W. but you will find far more inthustists to share tips with at www.woodworkingtalk.com

certainly strong enough, wood is wood whether new or old provided it is in good condition w/o rot or infestation.

I think you're building slab doors; it's typical to reinforce them from behind with a batten/cleat. If done nicely and detailed with chamfering or edge beads, it is perfectly acceptable, and better than a warped door.
Casey

A cleat on the back, or glue them to a piece of 1/2" plywood like a mosaic.

I was seeing a cleat in my mind.

I have faith in my joinery, but the wood I have questions about. Only a few pieces, but I had to use them for their color, grain, and some, paint circles.

I've also found conditioning the wood has made it seem far less fragile. I am absolutely loving how this is coming out.

I've been working on these cabinets for 5 years. First, building them because I had the materials, and buying a few when I had the money. Then I realized I could buy carcasses at the reuse centers for $5 and $10. Never could settle on a door I liked until I realized how in love I am with beaded inset.

So.... not only am I making beaded and regular inset, I'm using some of the cottage doors I bought (when I couldn't find something I liked) for a real, put-together look. I am not fond of perfect. It's more about balance and symmetry to me.

Thanks for the thoughts and suggestion, as usual.
Christine

Pictures!

I want PICTURES!

When I can get more than cell phone pics -- or at least good cell phone pics! I'm becoming very proud of these puppies.

I wish 1) my table saw's fence weren't broken, and 2) I weren't afraid of so much noise. The circular saw scares me, too, but I have a nice little one with the B&D Matrix system that I'm not so afraid of. Its jigsaw is also really cool!

"I weren't afraid of so much noise. "

Please buy sound proofing ear muffs, or you'll go as deaf as I am.

No worries. I hate noises. My first dance in Jr. High, I came home to get ear plugs because the music was too loud in 1970.

I have tinnitus from decades of working in loud bars. I wear my ear muffs when I vacuum. I hate the clanking of dishes, squeal of brakes, hammering, you name it. I have 3 sets different places in the house.

I find myself at the grocery store with them still around my neck. Talk about one weird cat lady...

Oh - I'm on my 2nd B&D Matrix saw. The first spins, but when I put it to wood, it stops. Amazon sent me a replacement, but today, less than 2 cabinet door cuts later, same thing. I've let it rest and am going to try again, but I'm thinking this particular attachment is a problem. Shame. It doesn't scare me so much.