Just want to start by saying thanks for the mod! The first point here is the only thing stopping me from including it in my server, but it would be great if any of the other stuff could be added in the future too.

Right clicking a dirt or grass slab/stair with a shovel should turn it into a path slab/stair, the same way vanilla paths are made.
In survival, it's currently impossible to make dirt path stair/slabs without silk touch.

Dirt Path stairs are the same height as regular stairs, giving a slight bump to the block when used with other path blocks.

Placing a block on top of a dirt path slab should turn it back into a dirt slab.

Dirt Slabs/Stairs should be able to have grass and mycellium spread to them. If spreading isn't able to mix blocks, then just having dirt slabs/stairs turn to grass or mycellium on random ticks if they are exposed to the sky and have a high light level would be close enough.

The grass textures don't match Quark's 'Greener Grass' feature, making them look a bit off when next to regular grass - probably worth looking in to when you add vertical slabs.

I will add that I am also having a problem with the slabs showing the correct texture. I use the Lithos 32x resource pack, and I love the look of the dirt paths. I grabbed this mod because I need dirt, path, and grass slabs and thought that they would/should match what is the default in my world... but they apparently don't. Please see attached.

I know I'm very late but first of all, thank you so much for all the feedback!

With today's 1.19 update I added the ability to harvest paths using Silk Touch, as well as turning grass slabs, stairs and carpets into their path variant by right clicking them with a shovel. Additionally I adjusted the drops of all the blocks (not carpets) to drop their dirt variant if not harvested with Silk Touch, to make the mod behave closer to vanilla.

As for all other issues and suggestions you mentioned, those were not as easy to fix or implement (with my beginner modding knowledge anyways!) but I will try to get to them as I go and learn more about modding.