To diagnose your problem will be very difficult as we do not have the gun in front is us. Before you install all of three upgrades, you meed to figure out what was the actual problem before you took it for repairs. Buying a plethora of upgrades I'm hopes of fixing the problem may not solve the issue, only masking it. You need to find out the source of your problem first. Try this:
Install all stock parts and see how it fires. If there is a problem there, diagnose and fix. Slowly swap out one part for another upgrade part and see how the gun performs.
The slide hanging when returning to battery is probably from the top of the hammer catching on the rear if the bbu. Your inability to fire less bbs can result from a dec I'm gas efficiency from the mag to the air nozzle, or heavier slide requiring more gas, or incorrect assembly of the floating valve I'm the nozzle, or the increase strength of your airsoft surgeon hammer spring resulting in more gas release per shot.
I would not sand or grind down the hop up rubber, as I never had the need to do this. I highly doubt tha the ratech bucking is much better than the nineball one, so you may want to reconsider. Based on your description, you modified and sanded a lot of parts which may be the culprit of all the issues you are seeing. I would suggest reverting all back to stock and starting all over to isolate the problem. With all the gas venting out from your description it seems like there is something wrong with your floating valve/nozzle, or your magazine gas route isn't aligned with the nozzle.
Unfortunately, without the gun at hand, and with all these upgrade parts installed, it is very difficulty to diagnose your issue(s).