Identity of the caterpillar is a bit uncertain, but definitely not monarch -- those things are brilliantly striped from the earliest stages. You are right. I have (and posted) images of the monarch caterpillar and they are colorful.
Side view to show the tail if any would nail down if it's a Sphingid. - I will see if it is still there and get a few more images.
BTW, it looks like there's one other small bug in there, crawling around, just a hair below dead center in the image. - Yep, that is the one I could not ID. Unfortunately, this is deep within my tomato plants and hard to get to.