@Urian:
Blackbeard original plan wasn´t capture Ace and make all this reaction. His original plan was taking someone with a bounty of 100 milions or more and show his corpse to the World Government for a position in the Shichibukai.
In other words, the main victim of his plans isn´t Whitebeard, the main victims are the Shichibukai.
But this is all assuming Ace hadn't been the one who was brought in by Blackbeard. That's like saying he wouldn't have tried to aim for a Shichibukai position had Crocodile never lost his. We don't know what would've happened so you can't really make a valid argument using that as a base.
Blackbeard's original target was Luffy, but he decided to settle for Ace since he has enough notoriety for Blackbeard to replace Crocodile. Simple fact is things change, and bringing Ace in draws Whitebeard to a position where Blackbeard can take his position like Shanks stated back before Thriller Bark.
We know that the Yonkou has 0 relationship between them and they are rivals but the Shichibukai are another thing, they have made a pact and through this pact they can´t attack the others and they can be called by the WG in every moment. The power of the Shichibukai is equaled to the entire HQ Navy when they are together.
I doubt seven individuals match toe to toe with an organization as large as Marine HQ. I'm one that thinks the Shichibukai, while a major force that if tampered with cause an unbalance on the World Government's side, aren't exactly an equal. These pirates are under the employ of the World Government so I can't really imagine them on the same level as the entire Marine HQ.
The difference between them is simple, the Navy has a complex structure of people and the Shichibukai are mainly 7. For example if someones kills and admiral the next day they will take another vice-admiral and they will give him the position of admiral. But with the Shichibukai findind a replacement isn´t easy. Now imagine the damage that BB can do if he destroy the Shichibukais with the exception of a few of them. With the simple destruction of the Shichibukai as an organization then BB will have a power equal to the WG and become the pirate king with almost no opposition.
Blackbeard's aim isn't to take down the World Government though, it's to become Pirate King. He's using the position to receive leverage regarding what people are on his side so as to not become overwhelmed by Whitebeard or any other opposing forces.
If Blackbeard took out the Shichibukai, the advantage is on the Emperors' side so he'd have Whitbeard and his crew after him with no major power as an ally. That would actually be a hindrance to his plan of becoming Pirate King. Plus the fact that Marines would be sent after him for betraying them so that's both sides after Blackbeard. It actually brings even more opposition rather than eliminate it.
The other part is the war against Whitebeard. I believe that the old man will win the war thanks to the secret sabotaging of Blackbeard against the Shichibukai and Ace will be rescued but the WG will have huge damages until the point that perhaps they won´t recover and this will start the decline. If we combine this with the possible defeat of Kizaru at the hands of Luffy and the everyday more powerful revolutionaries of Dragon then the WG will be destroyed.
So Whitebeard wins…then what? How would the World Government being taken down stop the World's Strongest and his crew from going after Blackbeard? The power that's generally had the advantage are the Four Emperors so I fail to see how the opposite side being taken down changes anything. All that would mean is pirates will overrun everything which isn't a good thing since the majority of crews aren't like the Strawhats. All that would cause is more problems for him.
As I mentioned in the chapter thread, if Marine HQ and the World Government didn't think they could win this war Ace being publicly executed would have never happened. They're the ones who place such importance on the balance and even then, to knowingly do something that is basically asking for war means that they're confident in coming out as the winner. So I highly doubt there is a chance they'll lose.
EDIT:
@Ubiq:
It's questionable as to how useful some of them will be even if they show up. A sociopath like DoFlamingo might well decide that it's much more entertaining to hang back and make Marines turn on each other in the middle of the fight than it is to actually fight Whitebeard's people, especially if it ever becomes apparent that Whitebeard might actually win.
Considering how trustworthy most of the Shichibukai seem to be even under the best of circumstances, the World Government might as well offer a full pardon to Crocodile, throw in something to sweeten the deal, and send him out on the front lines along with anybody else in Impel Down that's willing to take such a deal. The chances of somebody like Crocodile actually holding to such a bargain and doing his job are about the same as somebody like DoFlamingo doing his.
Well I'm pretty sure they're not relying solely on the Shichibukai. The higher-ups no full well how untrustworthy these men are, but considering how big a thing this current situation is they know some of them will help out. I mean if their side loses the Shichibukai will basically lose their status, and it'd be in their best interest to help. From Donflamingo's conversation with Disco it seems like he'll take part in this war, Blackbeard will show since Whitebeard's the one coming and Kuma's the loyal one of the group.
As for what you suggested, what will happen exactly when the war actually begins there's no way to know. All of the above is simply speculation and I'm simply throwing out who would show and why. I'm not even thinking in an ideal view since I'm not suggesting all 7 Shichibukai will even arrive.
The only thing we can do is wait and see what transpires.
