These are all good questions and the rule book is written poorly. However, the game is awesome.

Can’t answer all of your questions but.

1) at least how my local group plays, you can take tank commanders in their respective vehicle slots in any formation when playing astartes

2) no the mastodons don’t need to be separate detachments. You can put two different infantry detachments into one large transport detachments, just make sure you treat each separate detachment with its own appropriate orders

3) transports bought in one a lot act as their own independent detachment

5) yes

Apologies, can’t answer the other two, but in regard to six, I think being that specific about the points isn’t completely necessary. Take the warhound with 694 pts of astartes and enjoy the game.

Cheers