If an object like this shows up in some exposures but not all, and in different places, it's most likely a satellite. A more aggressive pixel rejection method during stacking, usually takes care of it. If traces persist in the stacked image, you can try removing it from the stack.
Only if the object shows up at the exact same location in all exposures, would I be worried.
As for the focus issue: a Bahtinov mask together with streaming during focusing, or a motorised focuser controlled by your image capturing software, should improve this.
