Boy, I can't help you with most of those questions, but as for #5: I would suggest you call the manufacturer. They're the only ones who really would know the answer to that question, especially since it could easily be "yes" for one brand and "no" for another.

Poly does not touch up at all well. The entire floor will need to be sanded and recoated, and you will still see the line where you stopped at the threshold. In addition, some of these paint remover products or floor cleaning products that you have used will impair adhesion of the new to the old. If that happens, it may look OK at first, but then start peeling off in a couple of months. If that happens, the only recourse will be to sand to bare wood and start over completely. If you've ever used anything with silicone or orange oil in in on the floors, skip the intermediate peeling step and sand down to bare wood now.

If these were a natural or traditional stain colour, you could probably do a little filler quite easily. But....your floor is a gray tone and that makes this a little bit more difficult to pull off. And the other thing you want to be aware of...filler will crack and crumble over time and eventually fall out. There are many people who have used a little bit of a silicone based caulking...but that can have unintended consequences should a sand/refinish be required. Of course you are welcome to pop of the nosing, pull the planks that are an issue, drop in new planks and then finish as required (assuming these are sand/finish floors). If these are factory finished floors, then this just got a little easier.

NOOOOOOO! No steam! Never ever use steam with wood. The steam 'cooks' the polyurethane causing it to haze and crack. If you REALLY want to make these look better, bring in a hardwood flooring professional to attempt a 'buff and coat'. This is the ONLY option for these types of dents/scratches. And remember: factory finished hardwoods are NOTORIOUSLY difficult to get a buff and coat to 'stick'. It could make the problem worse (as in ruin the floors). If you are so desperate to make these issues 'look' better, then please do it properly = buff and coat. Which may not work. For $3/sf.