I'd try to eliminate the flexible hose if at all possible. You can get a threaded piece of copper pipe and solder it into your plumbing system if you have copper plumbing.
In a well designed system, all of the hoses, valves, etc should exceed the threshold for the TPR valve on the hot water heater. So maybe the hose you had in there wasn't rated for enough pressure, or your TPR valve isn't functioning (very dangerous, you should test it).
For the tank itself, don't worry about water coming out of it. It will work just fine without water in it (in fact, it will likely work better since you've increased the volume of air absorbing pressure fluctuations). To pressurize it, you have to remove all water pressure first (shutoff the water supply and open a faucet) and then pressurize it with a pump up to your water pressure PSI level. As long as the tank holds pressure with the water turned off, the bladder is still good, and you can reuse the same tank.
If you're worried about dirt in the water side of the tank, fill and drain it a few times to see if you can clean it out. Chunks of rubber will likely get stuck in an aerator or shower head which would need to be cleaned out, so better to do this now.
In terms of mounting, they can be mounted any direction. If mounted vertically on a fixed pipe (doesn't matter which end is up) then you don't even need support brackets.
While I hate plumbing, I do quite a lot for close friends and family. I don't know exactly what you are doing, but I know a couple of things, and I hope these will help. When you loosen the gland nut closest to the wall, you probably need to turn it clockwise. Normally when you loosen something, you usually go counter-clockwise. In this case the nut stays on the pipe, not the valve. Always you a back wrench like you were talking about to keep from twisting the pipe.
Here's a You Tube video that shows it. I hope that this helps and is not exactly what you have done before. Did I say I hate plumbing?
Best Answer
NO, the tubing is part of the valve and not a seperate removeable piece. These valves were common (because they were cheap) a few decades ago. I have seen many where the tubing broke where it is connected to the valve, causing a suprise flood; be careful messing with it.
If you need to tee the line you will need to do it a different way. I recommend soldering a male pipe thread adapter directly to the copper, then using a threaded tee.