It is safe, but it will rot out. It is used to prevent electrolysis, but that is pretty much it nowadays. Pex is more commonly used for water pipes.
No tobacco pipe is safe. Tobacco smoke contains poisons. Brass heats up and causes the tobacco to burn much hotter. We do not have information on what that does to the chemicals, but we suspect it is not a good thing. If you must smoke, we would suggest sticking with more conventional materials.
PVC pipes do not corrode, hence these are used for drinking water supply.
Because lead is hugely toxic, and can leach into the water, thereby poisoning whoever drinks it.
its kind of safe but be careful
Lowes.com or Homedepot.com have all you need to fix those little leaks in pipes. they carry testers for water pipes to gas pipes. However, when dealing with gas it is always safe to call the gas company.
No, you cannot join copper and galvy directly. A brass fitting needs to be between the two to prevent electrolysis. You could also use a special type of union that has a gasket between the two halves. It is called a Dielectric union. If this junction is between pipes on your water heater and the cold water supply line, you could use a fitting made for that that also is a back flow preventer (ie a check valve).
If the outside air temperature is above about 30 degrees F, then yes; if the temperature is below that, then you should open the faucet to a slow trickle of water. If the water is moving, it probably won't freeze in the pipes and break them.
Smoking of any type can cause lung irritation, asthma, and lung cancer.
Since most water is carried to houses through copper pipes, yeah.
50psi is a safe route. No problems there. the fittings can largely effect the pressure effect noticed once the water leaves the pipes.
no
Average is 4ft, building code requires water pipes to be at least 6ft deep to be well below frost line and safe from frezzing.