Yes, 3mf files have all the tweaks ready, but it’s always a good idea to double check. Let me know on printables if you have any issues.

90% infill would be a lot of plastic. Much more expensive than the plaster, but you do you. For info, the finished base with the plaster weights something like 350g and it’s a similar weight to the shade. PrusaSlicer give an estimate of the material use after slicing. I would recommend to do it only if the base would be at least 300g otherwise the lamp might not be very stable. Maybe you can optimize things by having 100% infill for a portion at the bottom (top of the model) and a 5% infill for the rest, to make the center of mass as low as possible.