: it wouldn't hurt to beef up the thin
: spots from the inside with patches, would it?

Selective patching of thin spots from the inside is fairly easy and a good place to practice.

: I noticed that one of the patches was lifting up a bit at its edge,

If the lifting problem is not severe you can force some epoxy under the edge. If you can peel the patch off with a knife, you need to start over. Sand any patched areas lightly before you start so that the new epoxy/polyester will stick.

: Also, I noticed that whoever did these patches did not
: sand afterwards- aren't you supposed to?

In your picture it looks like the patches need additional fill coats to cover the weave. After the fill & sand you can use some automotive glazing putty to help blend the edges.