Is hot tap water safe for cooking?
Unless your hot water tank is very close to your hot water tap, this is a very energy inefficient. As Cascabel notes it would be faster to boil water in a electric kettle first, and then pour it into the pan. Put the pan on the heat at the same time if you are really in a hurry
Hot water systems are normally hot enough (above 55°C, 130°F) to keep water borne nasties at bay, plus if you are on town supply water it will be chlorinated etc
Normally on the first few meters of hot water pipe are copper, then it switched to normal crimped plastic plumbing. This will vary depending on your local building codes
The rate at which pipes lose their heat would ensure it never sits in the danger area for long, not that I think this is a big issue for clean plain water
In general, modern copper pipes are not soldered, they are crimped using special hand tools
Old or non-renovated houses may still be 100% copper pipes that have been soldered. This poses no extra safety risks with just clean water in the pipes