Awesome. So, first, that 2oz can is exactly the paste stuff to which I was referring. The descriptions do not always match (particularly on the boat years to which they apply), I have found. But I have ordered through iBoats several times.
So, some tips on the spots (thanks for the pics--that makes this super easy). First, your white area will be much easier than your grey. Do that first.
On the white, I see maybe one scratch that might be deep enough for gel to stick in it. You need a wide enough gap that you can spread the peanut butter in there and so it will have spots on which to stick on either side. So several of these will need widening/deepening with the Dremel. But you are very close to the black, so I would recommend a very low speed and go slowly. Nick the black and you made more of a problem than you had at first...
On the sanding, you can certainly start with the 280. I found that when I used the plastic cover method I described above, the 280 was worthless (it was basically pretty smooth already) and I needed to start with 400 or 800. I also sanded more finely--to 1000 (and I think one time 2000) grit. That will minimize what you need to do with polish. But you definitely can go down to 800 and then polish, especially if you have a random orbital to use on it. (But careful for that black--if you nick that with the 280 grit, you will need to smooth it with all the rest and ultimately polish it--not tragic, but you made much more work for yourself).
Warning: after sanding with the 400, you will think, 'Oh, @#$@#, what have I gotten myself into. This looks like crap. This will never look right again. I hate tdonoughue.' This is a normal reaction. Keep going. You are wet sanding, so it spreads the stuff everywhere and it will start to dry on and make everything look horrible. Take a sponge between sandings and wipe things down. It will help your confidence, but also give you a better idea of if you missed a spot or need a touch more on a spot.
Your gray is going to be difficult. The two scratches below will be pretty easy (treat them just like above). On that edge is going to be dicey. Why? First, you need some spot onto which the gel can stick. So when you Dremel that, you are going to need a bit more and you are on a curved surface, which will make control an issue. You don't want to take away glass, just gelcoat. Then, when you go to put on the paste (after wiping down with acetone, remember), evening that out over the curve will be very difficult. And you will have put the hardener in, so you only have so much time (~15 min) before it starts to get goopy and less workable. My recommendation on that one would be, before you start anything on it, find the plastic with which you will cover it. Ideally, you need something that is relatively stiff, the same curve as the hull there, and big enough to cover the whole repair. I know; I'm describing a unicorn. But if you can find something like that first, you will get to this part, slap on enough paste to fill and a bit more and then that piece of plastic will do the rest to get you into the basic shape you need.
Sanding on that edge is also going to be fun. You will be using a block, but that will mean that as you come around that curve you may leave a flat spot. Do not be tempted to sand this without a block. In fact, better you should try a curved block than to just use your fingers (you will put a dip in the new gel immediately). Sand lightly. You can always sand more, but sanding less is reeeeaally difficult. Note that your most difficult sanding will be with the bigger grit (280). Once you are down to something like 1000, the paper will be too weak to do as much damage. And once you get to polishing you are home free.
Home something there is helpful.
That was a safer place to learn, and yes the scariest part about wet sanding is that it looks really bad before it looks really good. The black are you mention shouldn't be a problem as that is not a black stripe, just a shadow being cast on the white. That entire part of the hull is indeed white. I'm going to be picking up a Random Orbital buffer as well that I can use for the final polishing, as well as to use in the future when waxing the boat and my vehicles. 
