If water is not kept at pressure in a house plumbing, will the copper fittings and the tubing become weakened?
Answers:
No, under pressurization will not hurt the tubing at all. Overpressuration and water hammer ( the rattling you hear when you close the valve too fast ) will harm it, the tubing comes in several wall thicknesses, but the tubing that is used as a standard will sustain a pressure of over 600 PSI, the weakness is in how well the joints were assembled by the plumber that did the piping, and the age of the material, everything that will rust or oxidize will weaken over time. Not to worry the copper will outlast the building.
Other answers:
Not likely
Not likely
no
If water is underpressure then no problems.
If water is overpressure then maybe problems.
No...unless it fluctuates frequently and with a high magnitude. The cyclic stress could eventually damage the joints. Similar to water hammer, though less abrupt.
If it's a consistent low pressure, however, no.