Suggestions on how to repair small section without removing entiwalkwa

You just need replacement wood of the same dimensions as the current wood. Pry the rotted pieces out and use the same anchoring method to install the new wood.

Remove the pieces, level the ground underneath and pour in Quickcrete. If you like and it is a large area, you can put in a paver and Quickcrete around it.

Looks like the wood was never sealed so it rotted. I know you don't want to replace the walkway. I hope someone else can give you an idea sorry I couldn't help

You could just try to take out the rotted piece and replace with pressure treated in the same size

I would buy some cheap edging to slide in to retain filler before replacing the board.

That is a fall hazard and could cost you alot more if someone takes a fall and injures themselves, not to mention if your homeowners insurance sees this.... you're paying more for it being there.

The wood is rotten, the stone surface is uneven, I would tear it all out and put in a safe walkway.

I guess you are talking about the beam of wood that is rotted. If you don’t want to replace the whole beam, go up a foot from the rotten end, and use a saw to cut that foot off. Clean up the wood and rotting wood. Build up the earth with more eart, and put in a good shovelful of small rocks, any kind or color. Now, measure back down to the crossbeam. Either, 1. Build a rectangular box to replace the missing beam by painting or staining the box to look like old wood, give at least two coats of water-resistant polyurethane, and setting it on the ground in place of the beam; or 2. Find or build a strong box of cardboard or thin wood, just a fraction wider than the missing beam, without the top cover which would be open to the sky. Fill the box with a fast drying concrete mix, mixed with a couple of cups of small rocks. You’ can also add a length of rebar to the box. Let the Quikcrete dry for a day or two, then tear off the rest of the box, and you have a concrete beam you can paint or stain as you wish. Now, the earth around the beams, even with rocks, and the “new” beams aren’t going to help all the rotting, drainage, mossy problems your retaining wall has, but you could get by a season or two before you replace the whole wall. So best of luck. ☺️