An awful lot of water is locked in the ground anyway. This has very little affect overall. It barely touches what is entering the short term water cycle from the melting of Greenland (which was declared doomed a few years ago)

Maybe not melt, but heat and force the release of water rich volatiles. Then it is time for fractional distillation to separate the volatiles. Water might not be the only thing we would want

As the OP pointed out. Callisto is the easiest of the Jovian moons to visit. It is also ideal as a jumping off point for the rest of the Jovian moon. With predictions about the idea we will have put people on Mars by the 2100s. The idea of a semi-permanent or permanent base on Callisto being built in by 2200 is pretty likely

500? I mean it’s entirely possible in the next 200 or so years. Tunnelling is a pretty good industry for some nations, and is likely to only improve considering the protection an underground structure would offer on the Moon and Mars

And that would be 8 generations at worst. At the moment, major hurdle is getting a manned mission to Callisto