Hey Jay,
The blue game changers are tuned the same as a regular blues harp. But if you push in the slide, you get a 2nd blues harp. They are a half-step higher. So if you have a C harp, pushing in the slide gives you a Db harp.
You can use these to just reduce the number of harps you carry, but the best way to use them is to “steal” notes from the slide-in “harp”.
Middle octave is identical to Solo tuning, which is the standard tuning for a chromatic harmonica. So if you focus on the middle octave, eventually you can transfer that learning to a regular chromatic (or the yellow solo tuned Trochilus, if you want).
As for relying on tabs - it really depends on your goals. If you want to pick out a tune once in awhile, there’s nothing wrong with tabs. If you want to become a musician, it is very much worth your time to learn to read music. Many ways to go about this, but the main thing is 1) use materials designed to help you learn to read music. Get some that go very, very slow. Because it takes a little more effort to learn to read music on harp because you can’t see the notes. 2) Work with a teacher who can read music.
If you do that and get good at it, it will open doors you never imagined you could walk through. I know it has for me.