How do I begin to mend this old wool Pendleton sweater?

The nice thing about sticky wool is that it is slower and more difficult to unravel. This sweater looks like a good candidate for using a crochet hook or latch hook to ladder up those dropped stitches and then all you need to do is secure everything together. Here is a video that shows one way to go about it. https://youtu.be/e6ZiAtBWpmM?si=Y2FmlHHoRfA1alqm

A couple of those are too big to ladder. But Swiss darning should work!

Look up swiss darning and get a wool yarn in a similar or slightly lighter weight (thickness) to the yarn that your sweater is made of.