I use the spreadsheet to hold dimensions/variable and formulas, makes it easier to modify designs. If I want to change a length, hole size height etc just change the spreadsheet values don't have to mess around with the sketches or 3d part designs directly.
One slightly frustrating thing i found is sometimes it fails to recompute the design or processes it incorrectly when values are changed by large amounts, that's probably more to do with how I design things though
I have considered adding windows to grub, but these days I hardly boot into windows so there is probably not much point.
I have seen both issues, when the sketcher starts to play up sometime I add more constraints to try to "anchor" a tangent in a certain direction but it can make the sketch messy The most reliable way I've found to stop it is small incremental changes e.g. say I want the length of a part to change my 40mm I'll change the size in 5mm increments allowing FC to recompute the model between each step. It's fiddly and frustrating but I can live with it, once you get to understand it's quirks Freecad can be very useful
I keep Linux and windows on separate disks, grub or windows boot manager don't know about each other. I have the Linux disk as the primary boot, if I need to boot into windows i use the bios boot selection screen. It's a bit of a pain at times(have to mash F12 to get the bios boot menu) bit it's less of a headache than trying to fix grub