Most 1 & 2 story houses with the boiler in a basement don't need more than 12-13 psi. (13 psi is enough to lift water 30' above the gauge. Baseboards on the second floor are typically only~ 25' above the boiler.) The standard pressure relief valves shipped with most new boilers open up a 30 psi, and with a cold/tepid boiler you're already above 20psi. When the boiler fires the pressure will increase, and if the expansion tank was pre-charged to 12-15psi it won't necessarily be able to accommodate the expansion of the water as it goes from 100F to 150F+.
Bleed the system water down to about 12 psi.
Tap both the plumbed side and the air valve side of the expansion tank. If it doesn't sound perceptibly different, the diaphragm is probably shot and it may need to be replaced. The air-valve side should have noticeable "ding" rather than a thud. If it thuds, use a screwdriver to push in the air valve a bit. If it hisses air it may just need to be properly charged. If it dribbles water it needs to be replaced.
The vent outflow of the backflow preventer is currently directed at an electrical device (an aquastat, perhaps?), which is an absolute gonzo-stupid way to install it, since if/when it drips it's dripping onto wiring creating a potential electrical hazard when wet. Installing a PLUG in the vent compounds the stupidity, since if the plumbing at that point doesn't relieve the pressure spike the pressure will be even higher, and something else will break first. The "right" way to resolve that would have been to install plumbing on that port directing the outflow down to the floor (at worst), or into a drain or sump (better). I'm not sure which idiot is worse- the first installer might have simply been absent minded or careless, but the one who installed the plug created a different type of danger. (Maybe they both should find some other line of work.)
The backflow preventer might be just fine, assuming you have relatively high pressure on the potable plumbing in your house. If the pipes tend to rattle a bit when turning off a faucet rapidly, high pressure spikes will propagate throughout the system, possibly exceeding the 175psi opening pressure to the vent for a few milliseconds at a time. The solution would be to lower the house pressure with a pressure reducing valve at the meter, and/or to install a "water hammer arrester" in a few locations near rapid turn off taps (such as washing machines & dishwashers, which tend to turn off the water extremely rapidly.)