Could be. :) Or maybe there never was any more to the deal than what was reported.
I agreed mostly with your first post in that we haven't received the whole story from Semi K, and that there is an Adder Horse connection of which we do not really know the full extent, not with certainty, at any rate.
I also agree, but the idea that the Horses are going to wait a half century to make some great betrayal just doesn't work for me. It's a deal that can't be upheld, and a secret that couldn't be kept even if it was.
I do think the Adders had an agenda. Put simply, they needed the Horses' cooperation to defeat the Burrocks. Going off into the Deep Periphery to chase down the Burrocks just wasn't on their agenda, and the Adders, when they made the deal, had no idea where or how extensive the problem was. Contracting the Horses let them keep their focus where it belonged. It closed off a potential avenue of retreat (what if the Burrocks had retreated to the Inner Sphere?), without the Adders having to "taint" themselves by chasing them into the IS. And if all those Adder abtkha the Horses netted were liberally salted with Watch agents, well, so much the better.
And, yes, I think the Sharks might have had something going with the Adders, too. After being "chastised" by the Adders and Vipers, the Sharks basically sat around in Clan space for several years before being given the boot. The battles they did fight actually didn't go half bad for them; and their second line was fighting the Vipers' front line IIRC. When they did get massacred, it was at Salonika (by the Burrocks, in a weird battle I've questioned before), and then at Vinton (by the Coyotes/Society).
The Sharks were on pretty good terms with all the clans that would survive. They were friendly rivals with the Adders, the two shared a strong mutual respect. They had active trade deals and were weakly allied with the Coyotes, Cloud Cobras, Blood Spirits, and Hell's Horses. The only clan that they didn't get along with were the
Suckers Steel Vipers. Now, these weren't strong alliances, and any of these clans would have sold even a stronger ally out to survive the Wars of Reaving. But it does open another possible avenue of speculation.
Again, I'm not a fan of a "centuries of treachery" plot. It just doesn't seem possible, and none of the clans involved are the kind who would trade tangible concessions up front for some vaguely specified but very expensive favor deferred into the far-off future. I just don't believe it.
OTOH, the Sharks and Adders are both long-term thinkers. Some kind of long-term arrangement isn't beyond the realm of possibility, as long as it benefits both partners. The events as they happened might also be explained by a short-term arrangement during the war that gave the Sharks the breathing room they needed to move their entire clan (in exchange, presumably, for the Sharks not interfering with the Adders' strategy).