Yes, changing an existing PlusSpec material but then editing the wall with the wall tool will cause it to revert back to the original PlusSpec texture.
Best solution here is to then create your own material using the "Create Material" button within the PlusSpec material section. I just did a test with the brick wall using a built in Sketchup Texture (Brick_Antique_1) and it works well. The key is to ensure the texture is scaled correctly. You do this by giving it dimensions in the set up area.
A good way to ensure you get the dimension accurate is to create a fixed size test plane in Sketchup based on the "Tile Size" of 2000m x 500mm... this is the size that PlusSpec shows you a "tiled" version of the texture you are creating. Then when you size your new texture in the set up area use a vertical or horizontal measurement that gives you a tiled view matching the test plane in Sketchup.
This is one of these things that is a lot quicker to do than to try and explain in words. Once you've done it a couple of times it's a breeze.
A point to note here though is that "legally" you probably shouldn't use a built in Sketchup Texture to do this. You'll note that before you can use the "create" your material in the PlusSpec web based area you have to agree you have the legal right to use the texture. In the case of reusing a Sketchup texture you don't actually have the right to use it to create your own materials. I'm sure PlusSpec asks you to confirm you have the right to use the material as it is being created and stored on their servers and they will not want to incur any breach of copyright issues.
In reference to your other points, rest assured... I know Andrew and his Team are "On a Mission" to make PlusSpec a real competitor to "the big boys". What they have achieved over the last year or so is a real credit to them and the tools just get better with each release.