it's important to remember that the only thing that matters are the keys. wallet software gets used to construct keys,

I totally flubbed that explanation, by using wrong word.

s/keys/signatures

if you can't sign, u can't spend. only thing that matters is the signatures

You need to have the descriptor.

that's true. the descriptor points to the private sig, same way it points to the public address

I've never yet used a wallet that let you have multiple spending policies within a single wallet.

yah, because the sig construction follows the same pattern as the address construction.