Silicone is best used buried 20' deep in a landfill, unopened.

Use polyseamseal, its all the things that silicone is not.

Are you sure 100% silicone was used? Probably not. AS I recall, the directions for 100 silicone say it doesn't stick very well to silicone, not that it doesn't stick at all, or something to that effect.

Usually it's caulked with something that's easier to manage (Kwikseal, polyseamseal, or some other "Kitchen & Bath" caulk. These may be "siliconized" (which are easy to touch up), but that's different from "100 silicone" (which doesn't stick very well to silicone).

This means you probably can just touch it up, then wet your finger and smooth it a little bit as necessary.

"...we only want to redo this once." Hate to say it, but tub caulking, no matter what you use will last 2 to 4 years at best. Usually discoloration, mold, etc. is the reason for replacement rather than failure to adhere.

Ideally all the caulk should be replaced at once, but in your situation, replacing just the back part should be fine as long as you're careful in the corners.

Whether you use high quality silicone or acrylic latex doesn't imho really matter, although if you're going to do just the one wall, you'll probably want to use the same type as you used originally.

You really don't need or want to let things dry for days.(Too much time just dirties the area again). Remove all the old caulk, clean with alcohol, use a hair dryer to dry it. Recaulk immediately. Now comes the time for patience. Multiply the manufacturer's recommended drying time by at least 3 (or if it's one of the "4 hour" ones, wait two full days) before allowing it to become wet. Spraying with a bleach solution once a week - let it sit for 10 minutes, then rinse and dry - can help keep the mildew at bay. For a while.

Thanks very much for the feedback. My husband did indeed use silicone, and I am wondering how we adhere the new silicone to the corners of the old silicone at two joints? We've removed the small section (about 1.5 inches) and may just remove the whole strip. It is coming off quite well and it is so new. We really only want to do this twice (ha) and hopefully it will last a good 4 years or so.
Thanks again.

I should add, my husband used a backer rod behind as the space is a little bigger than we had planned. Our priority is not to have water damage, this was a big job to tile (his first time).
Thanks again.

If the caulk is really coming off easily, I'd do the whole thing at once. Removal of the old stuff takes far more time than the actual caulking and you don't know if more of the original job will fail pretty quickly.

It's hard to tell with 100% certainty from the picture, but the joint does not seem to be so wide as to require backer rod. (I'm not at all sure about using backer rod in a high moisture area, anyway). The probable reason for the failure of the caulked area is that it adhered to the backer rod but not, or at least not very much of it, to the tub and tile. If you can, I think you'd be better off leaving out the backer rod so you have greater area of contact where you most need it.

Fill the tub as high as possible with water, then stand in it when caulking.

You want to have as much load on the tube as it sees to load it down and enlarge the gap with any structural flex that is present.

Caulk compresses a lot better than it stretches.

The tile worker that fail to leave a decent gap do not help things.