However, I don’t see Tywin reacting to Rhaegar dishonoring his daughter and Aerys holding his daughter and his grandchildren hostage with fighting on his behalf, given how he reacted to Aerys holding Jaime hostage in OTL.
Political conspiracy by its very nature involves risk, but Renly took every opportunity to manage his risks. The Lyanna thing was one of them.
However, to tackle your questions:
I don’t think the plan was to have Margaery be his “mistress en titre,” any more than that was the plan of the Boleyns or the Seymours. Rather, I think the plan was to pop Margaery in Robert’s bed and then immediately push marriage.
The famously forgiving Robert, who wishes that Loras Tyrell was his son? No, anger from the Rebellion isn’t a factor for that particular Baratheon brother in the same way that it is for Stannis.
The age of Barra’s mother suggests otherwise.
Given that Robert’s major drive in AGOT is all about trying to recapture his lost youth, that doesn’t strike me as likely.
I think you have to keep in mind the timing and sequence of events: Renly had just crowned himself (after screwing up his pitch to Ned) when Robb Stark was acclaimed king, and Stannis had yet to release his public letter, let alone ask anyone beyond the Stormlands for support.
But despite that, Catelyn does go to Bitterbridge to negotiate with Renly and talks to Stannis as well.
The Hornwoods are significant players in Northern politics, but they don’t seem to be as big as the Manderlys and Boltons, not least because they’re not able to stop the Boltons and Manderlys from using them as a battleground in a contest between those two houses.
The Boltons have enough land and vassals to raise 3,000-4,000 men, although that’s a lot less than they used to have before the Starks raised up the Karstarks and the Manderlys and rewarded the Umbers, all from Bolton lands.
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