Plumbing help

Hopefully this is a simple one. Seems like a simple one. But I are a idiot, so I figured I’d ask here first. 

While dealing with a valve that decided it was time to be a spigot, I noticed that the pipe was pretty blocked with rust. Unfortunately, it is impossible to get anything back there to clean it. 

There are about 3 fingers between that wall and the opening—barely enough to fit the damn valve. So obviously, the easy answer here is to take that T off and just clean it. But I can’t get that off for the life of me. How do I get it off? On the top of it is another valve, and that’s the end of the line.  

FWIW, that long pipe goes about 2 feet total to an elbow. 

Bigger pipe wrench.

But how blocked is it? Unless you noticed not getting enough flow I'd be tempted to just replace the valve.

Looks like that is under a sink in a cabinet, yes?

Yes, under sink. 

It’s blocked bad—bad enough that if I don’t fix it tonight, I will tomorrow. Pretty low pressure on the hot water. 

How far back do you have to go to get beyond the galvanized pipe?  While not against code, nobody uses galvy pipe for water any more except maybe for tieing in a well pump.  If you can get back to where there is copper, cut the copper and sharkbite on some pex.  Remake it in pex with push on stops.  If you have galvy forever, then you would be better off with two pipe wrenches.  One on the pipe and one on the tee.  Water pump pliers just aren't going to cut it.  Just got a rough inspection today on a house where I ripped out everything and repiped the whole house with PVC and Pex.  Tore out almost $100 in cast, $150 in brass and copper water pipe, and another $30 or so in lead water and drain pipe.