91% Isopropyl alcohol is your friend, if the board is even recoverable at this point. It is hard enough to meaningfully clean under the plate of the usual mechanical keyboard, corsair's exposed aluminum plate boards like the K65 or K70 leave you even less access and visibility in there than most others.
You could try flushing the whole board with some Isopropyl and working any switches that may feel sticky while you're at it. It is usually pretty great at breaking down gunk, and evaporates a lot more quickly than most other cleaners, although it may still take many hours if it is in a tiny enclosed space. I usually wait at least 24 hours or more, even with 91% myself. Some tricks I like to help facilitate faster evaporation are to leave the board somewhere with a lot of ventilation, like under a fan, and maybe prop it vertically on one side so that instead of pooling, most excess liquid should run out from between the plate and PCB. Reversing it in a few hours may help to disperse remaining pockets of isoproypl so that it can evaporate more quickly.
If that doesn't work, you can try to desolder every single switch so you can get direct access to the PCB for targeted cleaning, or if you lack the patience, concern, tools and/or skills, and are otherwise just going to trash the board, it couldn't hurt to try putting it through the dishwasher and letting it dry a few days and/or flushing it with isopropyl again.
