Nope, you haven’t asked this before, friend! Really interesting question…
I am personally divided about whether or not Valka would be able to break Cloudjumper from the Bewilderbeast’s control. On one side, Valka is an incredible woman, a dragon lady through and through with a very unique bond toward the dragons. This bond would in theory be strong enough for her to break through to the bond and help Cloudjumper regain conscious control of himself.
However, I am actually more of leaning away from that answer. I very well imagine Valka does not manage to get through to Cloudjumper in such a scenario.
First of all, Valka is more dragon than human in many capacities. Valka has lived amongst these creatures for twenty years, and as is such has adopted their mannerisms and social interactions. She probably has even started thinking a bit more like a dragon than most homo sapiens should. Because Valka is so dragon-like, it might mean that she is not “different” enough to get through cognitively to Cloudjumper. One of the reasons Hiccup is successful in HTTYD 2, after all, is because he is a man who can span the gap between the two species. He has a strong bond to Toothless to connect to his dragon through the fog of being mind controlled, but at the same time, because he is human, he is distinct and non-dragonly enough to pull Toothless away from another dragon’s control.
But Valka would almost be too dragonly to be enough of an anchor for Cloudjumper to successfully pull him away. She might “blend in” too much cognitively to the other dragons so that they cannot use her as a distinct outside element apart from the Bewilderbeast and the other dragons. All dragons can fall under a Bewilderbeast’s control, so they need the help of a non-dragon to get them out from that mind control. Valka might not qualify enough to them in their mental representations of who she is.
Another problem is that, because Valka knows so much about dragons, she knows how the Bewilderbeast’s hive mind abilities work. She will have seen it in action with the alpha of her sanctuary, and she probably has NEVER seen a dragon break free of that. This knowledge is a set-back for her. It means that she mentally will not believe she can reach through to Cloudjumper even if she tries. Even if she tries, her knowledge will be a huge set-back and make her less likely to succeed. Hiccup has advantages she doesn’t: he has not been around Bewilderbeasts long enough to really have their control abilities ingrained in him… and he is so emotionally compromised that this sort of logic is not even in his head. But Valka would never get that grain of doubt erased in her head, and that could negatively tamper with how she reacts to possessed Cloudjumper. It would, as a result, not be as effective.
It is quite possible she could get Cloudjumper free, but I see some factors barring her way that mean, at least as I see it, that she would more likely than not not be successful. Valka, even though she is even more incredible around dragons that Hiccup, is someone I see failing in this endeavor, for her excessive knowledge ironically turns against her in this scenario. I would have zero issue or argument if someone wrote her getting through to Cloudjumper, but this is where I stand.