House Kurita attacks the Republic first in September 2785 when the Ronin Regiments under the command of General Daisuke Toyotomi strike at the Terran worlds of Lambrecht, Kervil, Telos IV, Lonestar, and Chaville. This is part of Operation Katana which aims to restore Dieron to the Combine as the 5th Military District. It is in direct competition with Operation Khopesh under the command of General Ramses Vron who strikes at the Lyran worlds of Trolloc Prime, Gram, Chimpaw, Kingtribel, and Shimosuwa. Their goal is to setup Vega as the 5th Military District of the Combine. Its a race folks and the first one to conquer their objective wins!
The Talithan Borderers were hit pretty badly by Operation Avalanche under the command of Captain-General Kenyon Marik but I think the Oliver Borderers got hit worse. 3 Talithan Borderers were destroyed by the massive Free Worlds League invasion while 1 withdrew to another world. However, 4 Oliver Borderers were destroyed by Marik and another 1 was forced to retreat.
Kurita needs every able regiment they can get in the war and while they always try to exploit mercs to the fullest did not ban them.
The transfer from Robert to Jennifer was on the canon timeline but his illness may not have been so natural in origin.
Shattered Dawn was an alternate universe where the Terran state did make it all the way up to the Clan invasion but for specifics stay closer to the 1st Succession War.
Chew on that and umm yes you owe someone money cause it was the Dragon and not the Eagle who struck first.