To answer your questions (as I’ve asked them too):
For #3, yes you set spindle speed in the tool definition, which is a bit odd because the speed shouldn’t be per tool but rather per material use-case, so hopefully that’ll get adjusted in some future version of MeshCam, but that’s something to bring up in that forum instead of this one.
For #4 12k RPM is the rated top-speed. I don’t know if it has a “ceiling” that it imposes or if it’ll try to go faster, but it’s probably not a good idea for the life of your spindle bearings to try to find out.
As others have mentioned in other threads, squealing/chatter is usually from rubbing the aluminum instead of cutting it well, so you probably need to adjust your feeds/speeds to respect the amount of lateral force the machine can put out, so that you’re properly loading your cutters.
Also, to reduce over-spray & mess, rather than spraying coolant on you might want to just try using a dropper bottle like this one to carefully apply coolant ahead of the cutter, directly where it’s needed. I use this kind of thing for cutting-oil quite frequently when drilling/tapping.