I had a frustrating, similar experience to you when I bought my house. Wasn't originally thinking about a pool at all and, had no idea how anything worked. In the end, it probably took a month+ of diligence to revive it. I think it hadn't been covered for at least 4 years and had feet worth of debris in the bottom. We raked and raked and it seemed never ending. Vaccumed. Bought the leaf-vac hose thingy. I noticed that in my pools case, when I felt like "nothing was left" with the leaf rake... I think actually, there was just like - a solid layer there

and disturbing it with the leaf-vac and better water flow (I replaced my pump), "loosened" what was down there and I started getting a bunch more out. Anyway you've already dived in and could tell the bottom was clear now, but I just wanted to mention that might have been what was happening with you - fixing the main drain helped to loosen the "layer" that felt like bottom.
Now, I see you got a new pump but did you ever check the filter sand? I was stuck like you with my rehab until I replaced my equipment (forced to since it failed!). I totally replaced it all, new filter and pump - and I dumped the sand out of the old one to see it and it was just sludge. There was no way it was being effective. I actually started seeing improvements, after that. It was still slow going (sand filters are slow at this part), but it got there. I never used floc but was VERY tempted. It might be worth it to check your sand out, and totally replace it if it's really old. I also experimented with DE but only after my water was clear and only for about a week - it removed the really fine stilty stuff I couldn't get out and I didn't feel the need for it after that.
Also, I don't have a main drain and just 1 skimmer and my pool went from black lagoon to sparkling - don't give up!
Also, what kind of pump do you have? Is it a VS? When I installed my new VS pump I was running it super high. I thought, heck yeah, let's run it high to really suck stuff up. It started clearing much quicker when I actually dropped the speed. It was explained to me by someone here to imagine a river - super fast water pushes things through whereas slower, it can get stuck easier. (I just saw your signature and you have single speed so not an option for you, thats okay)
Hi! I found this place after seeing it highly suggested everywhere I've looked so I hope you guys can offer us some advice :) My fiance and I just purchased our first home. It's just what we wanted and it also had a pool, which we weren't looking for but always wanted. But, we have NO clue...
www.troublefreepool.com
There is my thread if you'd like to compare!