1. I would assess the situation on your "optional" areas to work on. If it is wood glassed over, you might drill some pilot holes and see if the wood is wet. If it is wet, I'd replace it. If not, drill a few extra small holes and inject CPES in there to help plasticize the wood and help aid against future rot.
2. Yes, grinding sucks! Wear a tyvek suit and use a good respirator. I also would wear latex gloves and tape the seam at my wrist. I found the 24 and 36 grit sanding discs worked best for me. I went through around 50 of them. Make sure wherever you are going to glass you get the tinted resin off and I would suggest grinding past any glass that was laid atop the hull from the original stringers.
3. Measure like crazy and think about how you're going to ensure you get things put back in right. A you can see in my thread, I did the stringers one at a time. That helped me ensure I got them placed in the right spot and to the right height for the floor. Try to cut stringers out carefully and as close to the hull as possible so you can use them as a template for cutting the new ones. I would recommend using doug fir and putting CPES on every piece of wood you put in there. I would also recommend using epoxy resin since it will be less likely to let water in. If you're going to foam, I'd probably build the framework without drains. That seems to be the general consensus on another forum I'm on with guys that have a lot of experience with rebuilds.
Also, keep yourself a good stock of beer so you can spend some time sipping a beer and staring into the boat to think about your next steps.
As far as working on it, I would definitely try to dedicate as much time as you can. It does take a lot of hours to get it done, but if you constantly put in a few hours here and there it progresses along very well.