Wow. That's a bummer. To help you locate the problem, consider that it's unlikely internal to the head, or perhaps the head gasket at all. If that much coolant leaked into the crankcase, it didn't likely go through the cylinder(s), even over 10 days sitting. I'm not saying it can't happen, just that it's much less likely than other suspects. Hydro-locked engines either don't crank, or bend/break rods if it does. With an additional gallon of (stuff) in the crankcase, it should have started an ran, albeit rough. More fluid than that in there, and it can begin blowing seals and breaking stuff. Perhaps it's a good thing it didn't start (you still have your original problem, I think, and they're not likely related).

I'd start with pulling all the glow plugs and pressurize the cooling system. Checking inside the oil filler cap and pipe can indicate how long it may have been in progress (a clean pipe and cap means it didn't run long with coolant in the crankcase). Also check the crankcase vents (little square covers on the valve covers) and draft tube. Just "wet" with coolant indicates very recent, probably before running any significant time, and not up to temp, while foamy white means it's had time and heat cycles to vaporize and condense. You need to find where the coolant came from, and the path it took. The water pump is rarely suspect, and has a double-seal at the shaft. The engine oil cooler can swap fluids. It's less common of a failure, but it's a first suspect, in this case.

Good luck, and keep us in the loop.