Changing Eclipse to a Tap/Hold ability to let the player decide on the buff they want is, of course, the easiest way to resolve the issue. For the helmith the bonuses could simply be reduced, similar to subsumed Roar. I think these are perfectly reasonable options.
That being said, a somewhat more in-depth rework to Mirage would also be somewhat nice. Getting some interplay between her abilities could be pretty fun, since she's pretty old.. And that interplay could self-balance the subsumed version of Eclipse by locking some functionality behind another skill; similar to how subsumed Pillage can't make use of the Blazing Pillage augment due to no other frame having access to Haven. Probably not worth the extra dev time though.