There is this kind of tube that prevent me to use any kind of wrench to unscrew the nut in it. But without unscrewing it, I cannot get the faucet out of its place at all. Is there a simple trick, or tool, that would allow me to go up there unscrew it. That tube is ~ 5 to 6 inch long.
You need an extremely deep socket. Most faucet manufacturers make them, and they're often included with a new faucet. That metal collar makes it look like a standard basin wrench won't work, but it's worth a shot if you have one.
I have a universal faucet wrench but it may be too thick because of the shroud, my next set of tools is actually a set of sockets that looks like a chain saw wrench maybe 6 sizes they are hollow and each one is 2 sizes. These may be long enough to be able to get a wrench on , I will try to find a proper description and edit the answer. I found part number 34a498 socket set on granger & amazon that is similar , this is not heavy duty but has gotten some nuts out in tight spaces like you have.