Generally, prerequisites just need to be completed prior to matriculation. So you’d be fine registering for it in spring and applying prior to taking the class.
You can apply before you have fulfilled all prerequisite classes. As far as explaining goes, you would do that on your secondary applications, most of them have a question along the lines of "what are your upcoming plans before matriculation" or "what other classes will you be taking before starting at our school" this is where you would say you are going to finish up your prerequisite classes (assuming you are going to take a physics course with lab). Otherwise you need to contact the schools you are applying to and ask them if your class fulfills their requirements.
It’s assumed. You don’t need to put it in on AMCAS, though I suppose you could list it as a future course in the coursework section if it allows you to do so.
You can look into a nearby public 4-year university and see if they hold physics lab courses in the summer. Those classes are usually available to the general public (but at a higher tuition). Completing in the summer is better so that you can send updates transcripts to the schools that will accept them before most II decisions are made.
AMCAS doesn’t care about future coursework. However, medical schools want to know your plans and will ask you if you intend to complete requirements.
My advice is for you to Enroll in a class by the end of June (even if you haven’t started the class yet) just so that you can accurately state on all of your secondaries that you have enrolled in a physics lab course that will be finished by matriculation.