Lady Ella

I don't think Ned would have been worried about offending Robert. After all, Catelyn's the one who's concerned about that when Robert offers Ned the Handship. Ned doesn't consider Robert a potential threat. I think Ned would just have said that he wants to keep his family together after what happened to his own father and siblings. Given that he hasn't fostered any of his trueborn kids, that's a plausible reason. However, Robert would never offer to take Jon himself. It would be a serious affront to House Tully by ignoring Ned's trueborn kids by Catelyn. The Tullys (especially Catelyn) would wonder whether Robert would try to make Jon heir to Winterfell. That's also another reason Ned could give for refusing. Ned might have been willing to foster Jon somewhere else in the south, though. That would be one way of getting him out of Catelyn's way and wouldn't have threatened his trueborn kids. It depends where, though. The Vale would probably have been Ned's preference.

I think Varys is right: power does reside where people believe it does. I think he understands that the answer is going to depend on the person, though, he simply believes he knows how to make everyone believe that power resides with F/Aegon. I suspect he's not as clever as he thinks he is. People will believe what they want to believe. Doran Martell will probably believe F/Aegon is Elia's son because he will want to believe it. Dany won't believe it because she'll want the throne for herself. Everyone else will believe whatever suits them best, which will divide Targaryen support. Power resides more with dynasties than with individuals, so I disagree completely that Robb's power base was shaky from the beginning - quite the opposite. His crowning was less to do with him and more to do with House Stark. The northern lords and even the river lords acclaimed him because they believed that power resided with the Starks more so than with the divided House Baratheon. For the majority of them (especially the northern lords), I don't think their opinion has changed. This is also why I think both F/Aegon and Dany will ultimately fail regardless of who people believe: House Targaryen is a failed dynasty, which is why they lost the Rebellion and Viserys and Dany lived as beggars in exile without anyone to help them. Since loyalties are divided, the Starks were actually right to reclaim their own crown because there simply cannot be one king in Westeros at the moment. It's the ones who claim to rule land where nobody calls them king that don't understand the nature of power.

I think the older houses probably did look down on the Freys to a certain extent as they considered them upstarts, but clearly a lot of powerful houses married into House Frey anyway, so I think that Walder Frey imagined a lot of it or at least exaggerated it. I think the biggest problem is simply that Walder Frey had so many marriages to arrange. At any one time he would have had several children/grandchildren who needed to be married off, so it's a never-ending task. Naturally, not everyone he contacts is going to agree to a marriage pact, especially as other lords have far fewer children to marry off and therefore have to think more carefully about which alliances they need. Walder was probably getting rejections all the time, which made it seem as though all the other houses looked down on them.