(1/2) on the subject of renly, do you think that if he had said that he wanted an alliance with robb (and allowed the north to secede like they wanted) he would have been a "good" ally for robb to have? i mean catelyn herself had doubts about him &
(2/2) thought his campaign for the kingship was illegal so wouldn’t robb’s lords and counsel also have the same issues?
Nah, the only faction Renly would have been good for is the Tyrells, for the obvious personal reasons and the fact they’re providing the means for him to take the throne by brute force. Unless someone or something actively engages Renly’s self-interest, he has absolutely no use for them, so he’d be a ridiculously unreliable ally. He’s vain beyond belief, so he wouldn’t think he needed additional help in winning the Iron Throne and certainly not at the cost of nearly half the continent. This Renly quote from ACOK sums his attitude up:
“The time for talk is done. Now we see who is stronger.”
He’s not remotely interested in making alliances, he’s only interested in taking his massive army and smashing everyone else into submission. Because what Renly is when all’s said and done, is a brutal thug in pretty silk clothes.
But let’s say he did offer an alliance on those terms…he’d still be a worthless ally, as far as fighting goes (and that’s all Robb would need him for, to defeat the Lannister regime). As a military leader, Renly is utterly incompetent. In the run up to the Battle That Never Happened, Renly is managing to do everything possible to lose it despite having far greater numbers than Stannis. He abandons his slow march to King’s Landing in a mad dash to confront his brother, leaving all his supplies behind, to the point “he must come to battle soon, or starve”. His battle plan is basically to throw men at Stannis until he breaks, and to this end he gives command of the van to not the most experienced or the best commander (like Randyll Tarly), but to Loras Tyrell out of pure nepotism. Meanwhile Stannis has chosen his ground with care, and is setting things up so Renly’s all-cavalry army will be charging towards him with the sun in their eyes. Renly’s response when informed of this? To press ahead with the charge anyway, despite being advised to attack before dawn. Why would Robb, who’s proven himself to be an excellent commander and who has won several battles, want to be saddled with this arrogant imbecile in a battle? Make no mistake, Renly wouldn’t be happy playing second fiddle in terms of overall command.
As for the Northern lords…well, they don’t give a damn about the Iron Throne or its claimants any more - the whole King in the North thing came about as a way of refusing to take any sides in the WO5K, so the legitimacy of Renly’s campaign wouldn’t be an issue. What would be an issue is that as far as they’re concerned Robb’s King in the North, they’re already independent and he does not need the seal of approval from a Southron pretender to a throne they have emphatically rejected since it took The Ned’s head.