Another mystery of the cosmos was what happened to our teak and holly when the previous owner had our IP27. When we looked at the boat last June, the damage was obvious as was some very recent attempts at a refinish job. "Issues with water" was the broker's answer, the surveyor had nothing more informative to add. I can see that new hoses were added, new clamps and a new electric modification to the head. However, there was no leaking head smell. In any event, the water made a mess of things and it looked terrible.
I started the refinish job last October. I have no idea what the factory uses on the sole, it must be iron varnish. The surveyor cautioned that it was veneer and to be gentle with the sander......right. I started using a 120 grit pad on my random orbit sander and got nowhere. I switched to 80 grit and finally started making progress. The finish is very hard and also quite thick so I ended up getting a 30-pack each of 80 and 120 grit hook and loop pads and using nearly all of them. It took a month of weekends to get everything sanded, I put down two coats of a satin polyurethane and it looks OK.
First, you need a decent sander, absolutely random orbit. You need to get EVERYTHING out of the boat. Even with a dust bag there was sanding dust on every surface, in every nook and cranny. You will need good dust masks, not the 99 cent ones, the really good ones with linings and valves or you will be sorry. You need patience which I did not have enough of, to get down through all that factory finish. Otherwise, when re-varnishing there will be light spots from the old finish. I have some light spots here and there but no desire to do this again anytime soon. Also, once you are done sanding expect to spend another weekend cleaning up all the dust or it will end up in your varnish. Like yours, not our entire sole was a mess but I could not figure out a way to only do parts of it. If there is such a thing as blending varnished surfaces I don't know how to do it. Lastly, it is likely that the water stained the teak, it will show up darker under the new varnish. I was not able to sand through the stain.
As far as the tank replacement, I can't help you with that, hopefully someone else can?