I'll address questions #1 & #4 together.
1)I assume it is not possible to create more tracks in mixer, or is it? Was the option of adding track discussed before?
4)On the "count in" it is not possible to use fills from other drum tracks, other than the ones in mixer, correct? Or it possible to use fill ins for the "COUNT INs" from other "real drums" other then the ones on the main style? If yes, what would be simplest way to do so?
BB is limited to a few tracks. However, if you save the project and reopen it in Real Band, you now have the original tracks created in BB and the ability to generate a whole bunch of real tracks to fill out the band. The cool thing is in BB, it always selects from tracks that fit the style. For example.... Hard Rock LA style will give you guitars, bass, and drums. But suppose you want fiddle and steel guitar? In BB, you can replace something with something else, but in Real Band (RB) you can simply add those to the project, and they play exceptionally well. I do this all the time.
Next.... if you regen the project, on occasion, it will pick up some lead in notes. However, you can always use your imagination and add some measures to let certain instruments play "before the song starts" to give you lead in on one or more instruments. Note of caution: If you regen and it does give you some lead in notes that you really like..... BE SURE TO FREEZE that track. Because the next time you regen the project, it will regen the lead in and it will be gone and you'll probably never get it back again. Freezing is the only way to save it..... well, you can also export it..... and probably a few other ways..... but you get my meaning.
I go a step further, and move the tracks into Sonar where I can use all sorts of editing techniques to get lead ins, fills, and solos where I want them. I find it easier to work in this manner. Some others do too, and of course other folks don't. Use your imagination and think out of the box when you want to do something that doesn't appear to be a built in feature. If you can imagine it, there's a way to do it.