That's actually a heck of a stroke of good luck the installer is still around.

Let me get the picture straight in my head. The vertical side walls, meaning anything above the shallow end bottom , the deep end bottom, and any "angled" areas is actually a fiberglass? Then it transitions to gunite for the other areas I've mentioned?

If so, it sounds like hybrid I've never seen, having focused most of my pool career on vinyl. Most gunite pools I've seen are entirely gunite. Actually it's plaster that is the last layer giving it the white appearance you see. The gunite is sprayed onto a metal framework then the plaster is applied to that. But generically they are refered to as a Gunite Pool.

Because they're called Black Gunite doesn't automatically mean your pool is a gunite pool. It could very well be concrete, but I highly doubt that at this point.

The "sub drain" is something that's only installed when ground water exists, they are correct about that. It's an additional cost that would only be done, if the home owner requested it. For instance if they knew there was an extreme between the dry season and the wet season of the water table.

Just how much water does the pool lose? They have automated filling systems that are quite simple. A garden hose, hooked up to a float type system. When the water drops to a certain level the a valve is open and water is let into the pool. Sort of like the way a typical toilet tank is filled. It whacks out your chemicals to a certain extent, but that can be compensated for. Also they're good through the winter assuring the pool doesn't empty it self. Could be a hit on your water bill though.

Your "hill" is a tough call without seeing it, but it's obviously a potential drainage problem. My only thought is if the deck is shedding the water properly now a drainage line at the perimeter might actually end up putting more water behind that "long" wall. If anything make sure the drainage "kicks" the water away from either side of the pool (if possible). Get it out and away, rather then just off the deck and straight down underneath it. A "pipe" that's only preforated on the top and not the bottom and pitched properly.

In your first post you said the maindrain seemed sealed with a "screw in" plug. If your mean a threaded plug and not the black rubber "stopper" type then there's a good chance it was permanently sealed at some point for a reason. If the plug is properly glued in a pressure test might be able to be performed on the line. This involves cutting it away from the system (assuming it's still plumped into the suction end of your pump . A pressure test kit essentially is whatever can be rigged up to keep a garden hose sealed to the pipe (jacketed clamp usually) a cut of off or pet cock, and a pressure gauge. The pool is filled with water, and then pressure is applied with the PSI of your hose (8-12 pounds should do it), then the pet cock is closed. The gauge is after the pet cock, so it will register the pressure you've applied. If the gauge falls after a few minutes, then there's a leak, if you can't get pressure into the line, then there's a huge leak. A pressure test isn't to see how much pressure a line can take, but only if it can hold pressure. The problem with doing this on a main drain with an existing plug, is it's not known if the plug is leaking (when a small leak does occur). When this happens you have to dive with a bottle of "dye" to the main drain. Wait for the waters to calm and then squirt the dye near the plug to see if water is escaping from the plug. All the while someone has to be the top side making sure that a constant pressure is being put into the pipe to replace the escaping water, sort of regulating it.

You could do a dive, remove the plug and "see what happens". If the pump draws air when it previously didn't then there's a smaller sized leak in it. If it won't prime at all then there's a large leak. This is assuming the line is still plumbed into the suction end of the pump. If the line isn't plumbed into the main drain then you can still "see what happens". Does the pool lose more water then before the taking the plug out? Same? Less? (doubtful).

There are options even when the pool has water in it. They just take more experience, an ability to hold your breath, or scuba equipment and certification.

As far as the "cracks" go, if they are deep enough so as to make it through the plaster (assuming it is a gunite with plaster pool), any failure in a fix will result in the pool leaking again. The gunite itself is not watertight, it's the plaster that's the final and watertight seal. Are the cracks "hair line" meaning very very narrow like a hair on a white sink? Or are they cracks one could slide a piece of paper or cardboard into (1/8th to 1/4 inch)? Or larger ?Hairlines don't always mean water is escaping from the pool. Although the gunite is not totally "water tight" is pretty darn close and will retain some water that makes it through, just not large amounts.

The next question would be in a trouble shoot for a leak is: Does the pool lose more water when running? Or does it lose no water when not running? Losing more water when running vs. sitting idle would point to a two fold problem. One in the shell and one in the system. A return line (water going back to the pool) leak will not manifest itself in any kind of pump or filter problem (not priming, air in the lines etc etc). But the pool will lose water when the pump is running because the line is under a certain amount of pressure. With out that pressure the leak might not be large enough for water to escape. If it only loses water when the pump is on, then I would highly suspect a return line leak.

If this is the case then eliminating (at the pump) each line individually and either pressure testing them, or running the pool to see what happens. Just plugging them at the wall of the pool would only worse the problem because it would apply more pressure to an already leaking line. If you have a way of regulating the return line with valves at the system. Try running the pool off of each line indivdually and monitoring the water loss. More? Less? The same?, with each individual return line.

You could probably take a few of these basic and relativity easy (other then the dive and pressure test), methods in the remaining days of having the pool running, and possibly narrow done your areas of concern for next season.