A new WH is actual tested to 300psi, but 150psi is where the safety valve SHOULD open. Anything above 80psi is considered too high in a residential setting so that 150psi figure is usually at least twice the 'normal' supply pressure and was selected as a standard so it wasn't activated except in a true problem situation. Water pressure will generally be the same everywhere in the system at the same elevation. Elevation will change the pressure, which is why they often use water towers. Pressure changes at about 0.43 psi/foot of elevation change, so the pressure in your basement would be more than that in the second story shower.
If the pressure gauge you have has a second, peak reading hand, leave it attached say overnight and you'll know how high your pressure gets. If it doesn't, you may not be catching those peaks. The water expansion will only spike the pressure when you've used a bunch of hot water, then shut everything off. During that heating process, the water expands, and will raise the pressure unless something relieves or suppresses it (normally, an ET). The more hot water you need to heat, the size of the WH, and the change in temperature all affect how much volume change. It essentially only takes a miniscule amount of expansion to raise pressure, as the piping doesn't balloon. You may have some flexible supply lines, and those will slightly balloon (and wear, which is one reason why you want to minimize that), but once everything has ballooned up, the pressure will take off. If you do use some water during that heating process, it will immediately relieve the excess. If your PRV has a bypass valve in it, any time the internal pressure exceeds the external pressure, water can flow backwards, assuming there's no other check valve in the way. How high that is can vary, too. A PRV isn't magic, so it cannot regulate the internal pressure if it's coming from within the structure after it, and, a bypass is a useless commodity if the utility has put in a check valve, as there's no way for the water to move through the bypass...so, wouldn't be surprised if those become harder and harder to find as they do cost more and are not that useful any more.