We call white stuff "snow sh*t" and grey stuff "silver slop: your photo shows exactly why we do not GOOP ROOFS... White will peel easier than silver.
Silver slop dries like cement and will not sand. If it is silver, pick as well as you can. You can use a very stiff putty knife and a hammer to try and lift it. I have tried everything, including heat, chemicals and gasoline. It is asphalt based which you will see when/if you try to clean the debris.
The vents have to come off regardless but now may be the time to consider new roof metal. It would be easier and you will never get all that goop off. Have fun picking...
I needed to ice my hands after the three hours it took to get just one vent off. Ouch.
Just FYI: I found that hammering the edge of a chisel straight into the hard "goop" would create a dent that I could then attack with a stiff putty knife. Worked for me, anyway. I did consider a chainsaw or a blow torch, but thought better of it in the end.
Those screws are available in stainless steel, which I used over 6 years ago. Some people say they will not hold paint, but I have not had that problem. One of vikx's (I think?) tips is to put a wooden toothpick coated with glue or epoxy into each screw hole when replacing the screws so they will get a better bite into the old framing.
I agree with all of the others, you are asking the right questions and getting the right answers. A year (or more) from now and you'll have a complete, safe and unique little piece of rolling history that will make you justifiably proud of all the hard work. Scraped knuckles, blisters and almost lost fingers or thumbs will be forgotten when you take your first road trip, and have confidence that you did it all the right way.
Those do look like hex heads. Are you removing them by hand?
If so, you need to get a drill driver. I use the sub compact Makita which is quite small and light. (18 volt model XPH11ZB) but any cordless drill will work. I would not go less than 18 volts. You will need nutsetters that fit the hex heads (1/4") along with phillips and drill bits. A must have for building a trailer.
You need to inspect the floor edges to see if it needs repair. It should NOT feel spongy anywhere... You might be able to repair certain areas without a complete floor replacement. Check the sills and floor joists closely any place that feels spongy.
I say wait on sawing the floor until you know how bad it is. Some areas may be fine and you could piece the rest. If there is a metal belly, it is likely to have more rot than an open one.