4. The Navy is being held up, but by what? If whatever keeping them busy is not Dragon then this would be a great chance for his forces to do something. I would think that for a Rebel army, attacking and subsequently harming a majority of the peoples that the World Government is hellbent on protecting would be considered a great victory. If the rest of the Tenryu is like those we've seen so far then they are probably not very well liked by regular peoples, Dragon could rally a lot of support by attacking them.
5. We already got a hint at the end of Black vs Ace duel that it was the "trigger" for the great event yet to come. Shank and White Beard met for a reason, if I've learn anything from one piece it's that Oda rarely ever does anything without linking it to something else down the line. It is very possible for White Beard and Shank to team up and try to save Ace since Shank has ties with Luffy and White Beard would probably want to save his First Mate. That could be the thing that is keeping the Navy so busy that they won't worry about anything else unless it's a huge crisis.