If you keep the retaining wall low, you shouldn't have any problems. I have a somewhat similar problem with a slope. On two sides of my lot, I have a somewhat steep slope, about 5 feet high with a very small shelf at the top where the border fence is. There isn't enough space at the top for a fruit tree, so I dug into the slope and placed some retaining wall blocks to give me more space for the tree. I haven't used any cement or glue to keep the blocks in place, just tried to level the ground as much as possible. None are higher than 2-3 feet and the weight keeps them in place. When I water, some leaks out, but that's fine with me. I'll try to post a picture in a moment.

Here's a photo with what I did in the background. Apologies for the pokemon, it's an old photo. I didn't have many choices as I tend to clear out my photos pretty regularly.

I dug into the slope, added the blocks, then back filled with dirt to give me room for the trees, or in this case dragon fruit.

Edit: Fixed spell check errors.

That's a good idea, another option was to have small pockets like you've done around each tree. If I do this, I may make a solid wall just because of the conditions. I have a bunch of rocks from a renovation project I could use, but it will be a lot of work to haul them all up there. Might be easier just to build a CMU block wall, I could even add some mortar.

Around here, people use whatever they can to make the style of wall you are talking about, including old tree stumps.

I do some work with limestone around here (kentucky, US) for fencing/walls and other things and usually use temp logs or pvc pipe in the wall that I pull out after done to allow water to drain -drain holes. I especially would consider that if you are building the wall on bedrock. Don't just pile the rocks though unless you plan on using concrete as well. You want to simply stack them but stack one side behind the other and use smaller filler rocks pushed in the middle to tie them together. Since you don't have experience I'd build the wall with a cut away view like this:

/ ......|

/ ......|

---------

with the front tapered and the flat back leaned slightly back toward the hill kind of laying against the dirt that will be pushing against it.

Do the corners first then occasionally in the middle build up and then go back and do the in between parts and occasionally use a larger rock that can go across both front and back and over filler middle rocks to tie everything together

I can't see any problems using rocks for only a foot in height....and plugging the gaps.