1.) Can Stardust Dragon Negate Stardust Dragon Assault Mode when Stardust Dragon Assault Mode uses its effect?

2.) Can I use assault mode to tribute there stardust dragon to special summon my Stardust Dragon Assault Mode xD lol if I can that would be broken

3.) I can only use Stardust Dragon or Stardust Dragon Assault Mode's effect once on a chain right because it tributes itself?

For Example.

I have Stardust Dragon and my Opponent have Stardust Dragon Assault Mode. He attacks me then I activate my trap Mirror Force so he uses its effect to negate and destroy Mirror Force then I can use Stardust Dragon to Negate its effect right?

1. Yes it can
2. No
3. Yes

Train, You Can Use Your Opponent's Stardust Dragon to Tribute It For Assault Mode Activate unless the a card effect is restricting it(i.e. Mind Control).

the first and the third ones can be activated the second of course it cant if it could be done that card would be ban

In general, whenever a card tells you to tribute a monster, that monster has to be a monster you control. Otherwise, the card will specifically tell you that you can tribute monsters your opponent controls.