bottom job advice

I'm rehabilitating FS #1632 and working on the bottom. The plan is to take off the ablative bottom paint, sand to gell-coat, fair, apply Interprotect for a barrier coat and repaint.  I did this several years ago with an FJ of similar vintage, so while no expert I understand the basics.  (In that case, a turquoise boat with worn gel coat was overcoated with yellow oil house paint.)  
The detective work comes in with the sanding to gell coat.
The deck and cove stripe at least of this early 1970s Douglas Scot appears to be the classic turquoise/light blue of the era.  Below the ablative bottom paint is a coating of white; I THOUGHT that was another layer of old bottom paint, judging from the square flaking in sections. But where I started to sand below the white, and upon closer inspection, where the next laye ris revealed via sanding in scrapes and gouges, I'm not able to discern gell-coat. 
1.)  From other FS folks, is it likely that the bottom gell-coat is a different color from the cove stripe? 
2.) Has anyone seen gell coat flaking off in squares (similar to old, lead house paint)?   
3.) Assuming the while  is gel-coat AND it's in bad shape at points, would you sand it all off and put a barrier coat or only remove it in questionable areas, then repair by fairing?
Thanks for any advice.

This may be too late but...
1- The gelcoat in your photos is the white that is cracked and chipping in some spots. Your boat's hull was done in white gelcoat, the waterline stripe was painted on blue.
2- Yes that is what old gelcoat does especially so when applied thick.
3- Hard to say depends on what your use will be. Serious racing would need a lot of work. Daysailing not so much, maybe just enough to make it look good enough for your eye.