A1) If Anton Marik doesn't massacre the Dragoon dependents, he could win his civil war (or at least not lose it) and marry Max Liao's heir. This gives the Capellans stronger backing for the Fourth Succession War, and - without Thomas Marik in the line of succession - means the FWL has no incentive to back the FedCom and Dracs during the Clan War. Lots of things also change if Alessandro successfully assassinates Katrina.
If Max succeeded in assassinating the Duke of Tikonov during the 4th Succession War, the FedCom wouldn't have gained that region so easily; and if they fail to cut the Tikonov Commonality from Sian's control (and especially if Anton has been a stronger Liao ally) the Suns won't conquer the further parts of the Sarna commonality. Alternatively, if Justing Xiang becomes consort to the new Capellan ruler, the whole Confederation (instead of just St Ives) could become a Suns protectorate.
A2) The players should be aware of anything major their characters would be aware of when the campaign starts, and I think it's fair to offer a vague warning about the scope of other changes they might encounter as the campaign progresses. Probably important to describe the scope of the campaign and how much influence the players can expect to exert.
A3) Canonically, I think an Andurien/Betelgeus/Canopus block is the only viable candidate for changing the balance of power.
A4) If the Federated Suns could sway the An Ting region over to their side, like they did the Northwind Highlanders, they begin to cut the Combine off from its historic root in Galedon and New Samarkand. The Free Worlds could absorb Canopus or some Lyran duchy. It's easy to justify individual worlds or small duchies going one way or another on any border, but anything bigger - like Rasalhague, Tamar, Chesterton, New Syrtis - gets harder because of animosities, current momentum on the border, and specific personal ambitions.
A5) The pirates are lairing under a historic church or important industriplex the PCs' patron won't want damaged. The pirates are secretly supplying a worthy cause. They're all orphans from Penzance. The "pirates" aren't pirates at all but political dissidents or opponents of the PCs' sponsor. The pirates have personal connections to someone the PCs' like. The pirates are on a personal vendetta related or unrelated to the PCs' sponsor, where certain kinds of action will draw the PCs into a bigger mess between multiple players. The pirates are friendly or unfriendly POWs escaping fair or unfair treatment. The pirates possess knowledge - buried munitions depot, names of the PCs' secret political enemies, how to contact a good information broker - the PCs want. The PCs have family among the pirates. The PCs' sponsor(s) have family among the pirates. One of the pirates has a hereditary claim to the PCs' sponsor's title, lands or possessions. The pirates have leverage on the PCs. The pirates have leverage on the PCs' sponsor. The sponsor has leverage on the pirates. The PCs have a specific target they want the pirates to attack.
A6) The "DropShips & JumpShips" book, and the "Chaos March" sourcebook. The latter isn't the right time period but it's a fair survey of duchy-level activity across a small region.