What would have happened if the Reyne-Tarbeck rebellion would have had outer support? With this I mean that the Reynes had claims to Casterly Rock older than the Lannisters themselves. Supposing than Ellyn Reyne would have had younger sisters who would have been capable of marrying one a Tyrell and another a Tully (or a Redwyne), how much would that have changed the outcome and even the beginning of the rebellion? Tywin relied on speed after all to defeat them, but supposing the Tyrells or Redwynes or Tullys would have been willing to meddle on Westerlands affairs (The Tyrells being eager with having a wide support, the Redwynes being the monopoly on the sea-trade and the Tullys still hurt for the broken betrothal of Celia) so, just speculation, really, because a web of alliances (other than the Reyne-Tarbeck themselves) may have been essential. And now that I think of it… did the Gardeners marry some Reyne to try overthrow the Lannister regime in some age before the Draconic Conquest?

Also, is Ellyn Reyne supposed to be an alter-ego of Barbrey Dustin more than one from Cersei? Both wanting to be their overlord’s family above all, but ending up hating them more than nothing in Westeros? One in the North, the other in the West

I don’t know what you mean by “claims to Casterly Rock older than the Lannisters themselves.” If you’re talking about Loreon the Lion, not only is that claim very much on the distaff side, but it doesn’t predate the Lannisters. I couldn’t find any mention of an alliance between the Reynes and Casterlys, for example. 

As for outside support during the Rains, I think a house would have had to be reckless to the point of ambitious – Aegon V had already militarily intervened twice in the Westerlands, and there’s no reason he wouldn’t have gone for a third. But yes, it would have helped. 

I don’t think Ellyn Reyne is an alter-ego of anyone. As the song goes…