So if you use it on cast iron sewer line. It will be there as long as you kept the home? Let the other person deal with it.
If there is rust on the cast iron it coming from the inside out. The sewer gas rust the cast out and there nothing you can do than cut it out and replace it. If you get a...
Where is the 30ft at? If it Is the last 30ft at the tank then it your tank. Clogged or need to be pump out. If it the 30ft from the water closet the the pipe has a bow in it. Will need to be fix at that point. Hope it not the hole 30ft. Dig it up and see what going on. Not going to get better.
Sound like the pipe is back pitch under ground or broke. Call a plumber and have them camera the line then you will know what going on under the ground also see if the line is vented.
Only if you have a meter in the house. That must be jump from one side to other side. As long you have copper or gal water lines in the hole house. You be fine but if you but CPVC or pex in then you must up that spot because the ground was broken.
Before you cut holes in the wall. Look at the shower first. Check the grout around the shower around floor / walls if grout bad fix with new grout. Check the shower valve face plate. Pull it off and see it the gasket is good some times the water leaks there sometimes. Look at everything first be...
They sell supply tube at home depot. They also sell new faucet there too. You said the faucet 20 year old anyway. So if you want parts to fix the faucet go to a plumbing supply house. Or call a plumber and get rip off. Or call a handy man jack all trades master of none.
You could of went to home depot and got the plumbing 1,2,3 book. .
Clean fitting and copper
Flex the pipe and fitting
Heat up the fitting
Solder the joint
Don't worry about why it corroded just change it out and forget about it until it happens to the new one. Then when that one is corroded change that one. You on it all right. Jack of all trades master of none.
If you no that you are supposed to do it then why not do it. It will rust before the water heater will. The expansion will not work wright. The you will be doing it then.