IMO copper pipe is way better but too expensive now. I'm hoping to find some in a YS. I used to use copper on all the tealight holders I sold but then the price of copper went way up and I'm no longer selling them so have used rebar for both the tealight holders (same thing as the teacup birdfeeders but I put a tealight in mine) and the stick totems. For the copper pipe DH used to hammer the end so it was pointed. I think with the PVC it really would depend on how soft your soil is, more than with the other pipes. You might make a hole with something sturdier like rebar then put the PVC in especially if you have hard soil.

You can go to an extreme to try to cover all bases. There are several whole house water cutoff devices that can be installed. Two that come to mind are the flologic and watercop automatic valves. With these your remote water sensors can not only trigger an alarm code to your monitoring provider, but can also signal these devices to shut off all water to the house. With the right alarm (or more appropriately automation/security system) it could also take automated preemtive action and shut down the water line upon a power failure. All of this depends on everything happening right. What if the water sensor contacts get real dirty and are covered with a good layer of sticky dust/dirt? Will it still trigger? If your sensor is battery powered, did you remember to change the batteries on a scheduled basis? Where the leak occurs, will enough water reach the sensor to have it tigger? Etc, etc... The flologic seems interesting as it has some logic of it's own that "looks" for leaks. If it senses uncontrolled water flow beyond a period of time (which you set) it will trigger. I believe it can also be setup for some exception conditions and can be tied into a security/automation system where it automatically closes everything when you place you security system in the "away" state.

You have the wrong material. To the uninformed laymen copper pipe & tubing can be quite confusing. 10' or 20' lengths of rigid copper is referred to as "Hard Drawn Copper" while roll copper is referred to as "Annealed or Soft copper" The phrase "Type K, L, or M" refers to the wall thickness of the material. Now here is where it gets confusing. The copper used for water supply and distribution is properly called "Copper Pipe" and it is defined by the "ID" (internal dimension). All threaded fittings for copper pipe are made to the IPS (iron pipe standard) which means that the threads are the same size as the threads on iron pipe. The copper used for fixture supply lines,I.E. from an angle stop to a faucet, for gas lines and HVAC lines is "copper Tubing" and the size is defined by the "OD" (outside diameter) of the material. So to answer your question. You stated that the material you have is 1/2ID type L, therefore what you actually have is a 60' roll of 1/2" Medium wall copper pipe. (It could also be labeled as 5/8" OD copper tubing) whereas what you need is 1/2" Type L copper tubing. They make copper tubing that is specifically listed as approved for gas but most gas codes will also allow you to use TYPE ACR (Air conditioning & Refrigeration) copper. You will find that copper listed for GAS or ACR applications is a bit more expensive than copper pipe. This is because copper pipe is packed with a tape or slip on plastic cap on the ends to keep dirt and debris out of the pipe. On the other hand, GAS & ACR copper is certified dirt and moisture free and it will be packed with a rubber plug on the ends. When you pull the plug you will note a slight pressure release because the tubing is dehydrated, then filled with a light charge of nitrogen to prevent moisture from getting in the pipe during shipping & handling.

Pex has a slightly smaller diameter than copper, which is why there is this requirement. The valves are designed so that at maximum pressure, all of the water will flow to the tub spout. If pex is used, then it's possible that there will be slightly more pressure than the pex will be able to handle and that extra pressure could result in the extra water getting diverted up to the shower head. So the shower head may drip while you are filling the tub. That's the ramifications of your installation. It may not be an issue in your particular case since your specific pressure may not cause this effect, but it's in the instructions because it could possibly happen.