The GG&G Mossberg 590 Shockwave Side Saddle Shell Holder, 5-Shot carrier is capable of accommodating mini-shells as well as all other 12g ammunition.
Yes, they should be compatible.
No, the model for the 590 can be found here: https://www.opticsplanet.com/gg-g-mossberg-500-590-12ga-scope-mount-late-model-post-1997.html
GG&G does offer a different model that will fit the 20 gauge Shockwave, it can be found here: https://www.opticsplanet.com/gg-g-mossberg-500-590-12ga-scope-mount-late-model-post-1997.html
We are not able to confirm that, it will depend on weapon's configuration, size of the person, and type of sling being used.
No, this will not be compatible with the Crimson Trace laser due to the threaded pin that is used for mounting the laser.
Not at all. The directions are great and only involves a long trigger pin and a standard screwdriver screw. It is one of the easiest installs for the same type product.
No, this is only compatible with the Shockwave. The model for the 590A1 can be found here: https://www.opticsplanet.com/gg-g-mossberg-500-590-12ga-scope-mount-late-model-post-1997.html
GG&G states this saddle will not fit the 20ga Shockwave.