I also keep the drop checker in my 50 gallon tank on the side opposite where the CO2 enters and use two canister filters plus a Koralia 1 to move water around. I do move the drop checker to different locations from time to time to check circulation and see if the levels are consistent in different parts of the tank.
I am running 108 watts of T5HO over my tank, the 50 longs are not that tall and I found it difficult to get the tank dialed in with the light sitting on the legs. It is now hanging 13" above the tank, the tank is still difficult to keep dialed in, very easy to end up with staghorn algae if I am not careful. Every tank is different what works easily for one person may not work the same way for another. It is easier to learn the theory behind what should work than to apply it. This hobby has an ongoing learning curve, you learn from mistakes as well as the good choices. Remember that each tank is a unique constantly evolving eco-system.
Your problems seem to be related to having too much light in relation to your CO2 and dosing levels with the CO2 being the main problem. That is different than your 108 watts of T5HO being too much light to ever be able to make it work. Put a fixture with six 54 watt T5HO bulbs directly on your 50 gallon tank and it is very likely that it would simply be too much light to ever get the tank in balance. There is a limit to how much CO2 you can inject before you will kill your fish. Less light makes it easier that is all, but you still have to find the balance.
Check out the following thread about making a DIY ADA style light bar. I made mine, it was cheap and easy. The most expensive thing is the conduit bending tool, you may be able to borrow one. I bought one, bent my conduit and returned the tool to Home Depot, just make sure you don't remove any tags or scuff it up. Some folks just bent the conduit in the store without buying the tool. I made my light bars so they faced forward, you can see a picture of that style in post #69 of the thread, that style only requires one simple bend per bar. The photo in post #69 also shows how to attach the bars to the tank stand.