Yes, the grid in Inkscape is prone to weirdness. It is happy enough (usually) with SVG created in Inkscape, but gets confused by ones from other sources. The grid scale can end up wildly off, I do not know why. Sometimes exiting and reloading helps. The grid origin in Fritz is determined by the first pin (pin0) in your part, so in Inkscape your grid origin needs to be aligned to pin0. It usually is, but is worth checking too.
The pins in Inkscape are independent of those listed in the parts editor except that when you first load an SVG in the parts editor, IIRC it creates new pins for any it finds in the SVG. I don’t think it removes them. If you add pins in parts editor by increasing the count, it tends to leave a gap in the numbering between the original and the new, so if you go from six pins to eight, you get pin0 to pin5 and then pin7 to pin8. If you are not paying attention, you can then end up with pins in the two products that are linked but who’s id numbers do not match. For good practice, they should be the same. Fun, fun, fun! I tend to hand edit the xml a lot, to avoid or correct these funnies. Look at the part file format spec and learn it - it will help, a lot.