The vegetable oil he shows is only used to help seat the seal and it should only be used on the rubber piece. But I have never had to use a lubricant on the rubber part to seat the seal. I have seen other seal videos recommending other types of lubricant for the rubber so I think it is a SOP in the industry. Another trick to keep the seal clean is to lube the housing instead of the rubber.

Anyway as to your issues:

1) The impeller will normally stay on by itself without an impeller screw but the parts diagram shows one so it should have one. Parts

2&3) Sometimes that can be a real PITA but a strap wrench is the right tool for the job.

4) There should be 4 screws that hold on the rear of motor to the motor housing. If you pull on the back of the motor, does it separate from the housing? I would try to find some screws to fit.