So if they aren't going to be using the main engine for a major trajectory shift, does this mean that more fuel is available for the thrusters?
Maybe, maybe not ; for a number of possible reasons.
(1) Do the main and attitude drive systems use the same fuels? For the large number of anticipated firings for the attitude thrusters, you'd maybe go for reliably hypergolic fuel/ oxidiser couples (i.e., on contact, they ignite). But for the main engine with only a handful of planned firings you might choose a monoprop with electrical ignition. That question is readily amenable to research.
(2) Is there a cross-feed system from main thruster fuel tanks and pressurisation to attitude thruster fuel tanks and pressurisation? Or, to rephrase the same question - is there a mission-critical need for such a cross-feed? Because if the system isn't absolutely essential, it would have not been designed. Remember that every valve (not "most" but "every") and every joint in a pressurised fluid system is a potential leak point. Again, if you find the plumbing design drawings, it's the work of moments.
It'd be fantastic to get some really good pictures of Europa (life!)
Any life on Europa which isn't considerably more technologically advanced than ours is going to be on the underside of 30-50km of ice. Now, ice pictures can be pretty, but it's a monomineralic rock with a coarse grain due to regular resublimation and/ or refreezing. Even under crossed polars and thick sections, "dull" is going to come to mind. (OK - I've probably done a few hundred thousand rock descriptions more than you. I've a higher "dull" threshold than most people for rocks. But "dull" will come.
and Io (volcanoes!).
Now you're tempting me. But not strongly enough.
Or just put it in a relatively distant parking orbit around Jupiter and (because it's solar powered) let it monitor the Jovian system for (hopefully) decades
Nowhere near enough. Decades isn't remotely good enough. When we're talking about (potentially) wiping out the only other origin of life in the universe from our own
NASA would seem to agree with me - hence the dive into Jupiter.