I initially installed 64 bit flavor of 5.10 on my new system. I now believe this to be a mistake since synaptic prevents me from installing many apps. I 1f40 t's not fitting into the "it should just work" philosophy of Ubuntu?
Can I install 32 bit now and upgrade (not losing data) to 64 bit at some later release when it has better support?
Can I even install 32 bit on my 64 bit box?

I believe you can install the 32-bit version on a 64-bit processor. As for upgrade without data loss, you'll just need to create another partition on your drive to backup your data (your home folder should be all you need), and install the 64-bit version over the 32-bit. You can't upgrade directly though, so you'll have to manually copy your data back and reinstall all your applications.

I am not quite at home with hardware. I had presumed that a machine with 32-bit
processor can never use a 64-bit. Perhaps I got it all wrong.

It's running just fine. 1f40

you are right, a 64 bit operating system won't run on a 32 bit processor, but the reverse is possible. you can run a 32 bit OS on a 64 bit processor.