Yes, galvanised pipes may certainly be used. Making sure that the threaded joints are well sealed, however galvanised piping can be tricky and sometimes difficult to install. Copper pipe is so widely...
It is to drain residual liquid left in piping.
To keep the steam in the system and allow the cooler condensate to return to the boiler to be reheated and start the cycle all over again
if your waste pipe is PVC and your glue is for PVC . yes! but make sure to use a primer(cleaner) on the pipe first.
Type K copper
Out of Thin Air - CNET Loaded
Winterizing Sprinklers - Troubleshooting Ice Problems
Teeing Off Sprinkler Lines
Plumbing - Soldering Copper Pipes