Grapefruit can make a very nice melomel, but the acidity is an issue. It may surprise you, but the pH was not so much of a problem. With many grapefruit, the pH isn't below 3.0 (as opposed to lemons or sour oranges). It might not take that much potassium bicarb or calcium carbonate to adjust the pH up to a range to keep the yeast quite happy (around 3.4). I have even been able to ferment grapefruit without adjusting the pH, though I wouldn't necessarily recommend that.
There isn't a specific limit on how much potassium and calcium that can be added, but if excessive amounts are added you may get chalky flavors (in the case of calcium carbonate) or metallic/salty flavors from the potassium.
The problem with acidity in grapefruit comes in the flavor. If you use a lot of grapefruit you will have to balance the acidity (and the astringency) with lots and lots of residual sugar. I made a 100% grapefruit juice melomel once and it needed a gravity above 1.050 to be drinkable. If you want to make something that is a little lighter bodied, you have to keep the amount of juice down. For many winemaking recipes they recommend 6 grapefruit as the amount for one gallon (that would be something like 3 cups of juice). You can go a bit higher but if you exceed it by much, you won't have it drinkable as a dry mead and sweetening will be needed.
I don't think using the whole fruit is a good idea - there's too much bitterness. I think it is better to juice the grapefruit and use the juice. You can add some grapefruit zest to boost the aroma. Also, I would not heat the juice as grapefruit is loaded with pectin, and I would use pectic enzyme with it.
I hope you get a good batch.
Medsen