This is one of those counterintuitive building science things. Done properly, a house with a conditioned attic will require less HVAC energy than a house with a vented attic. Generally speaking, there's enough gains in latent loads, stack effect control, and keeping the ducts in conditioned space to offset the additional conditioned area.

but vented or conditioned is much less important than getting the details for either correct.

This, plus there's often regulations around setbacks and zoning that can come into play. Many areas allow an uncovered deck to extend into required side yards or rear yards. but once you put a roof on it, you have to stick to the setbacks.

I'm not going to say whether you should or shouldn't, but I will point out that model codes require something to prevent uplift due to wind, which deck blocks will not solve. Otherwise you've just built a giant heavy sail-projectile wrecking thing for the next tornado or wind shear. You may find that the work to sufficiently anchor it down to be close to the work required to put in some sort of actual foundation.

You can mess with the PEX if you want but the easiest way is just to buy a threaded brass T for the hot side and close/cap the appropriate valve on the cold side. Unless the valves are trash you don't even need to turn the main off.