Questions...

1) Yeah, it's a practice to make props that are sure to never be touched by players or really seen to be touched by projectiles non-solid. Just like nodraw though - you won't even notice if you miss some. Yes, better do that than not.
2) No, at all. These do not generate a rendered polygon and lighting isn't calculated for them.
3) You don't really use prop_detail - game does it for you. It's such props that is placed on some displacement surfaces (seen that grass?), it never interacts with lighting and collisions and is overall the chapest prop type, but it's not that intended to be used. VDC page.
4) You mean turn them off? There is a thing called tags, and most TF2 content is tagged with 'tf'. So, in the 2nd text field (the bottom one), type 'tf' and get only TF2 textures! Tell if you meant another thing.

1. at the very least it saves cpu time. The less stuff to consider for collisions, the faster the process is.
2. the tool materials have special flags that vbsp reads and converts into other flags that vrad among others use. It's smart enough to skip lightmaps where it doesn't make sense to have them.
3. Old artifact of the displacement detail system. Nearly useless.
4. Texture transformations can only be applied on individual faces.