10 pounds low on the rail fuel pressure is too low. Spec is often around 50, so 10 psi low is 20% low. Too much to ignore. That could cause misfires and rough idling. Perhaps your mechanic is thinking his fuel pressure gauge is reading a little on the low side, that’s possible. But if it is really 10 psi low, that’s got to be corrected before investigating other potential causes. It’s like if you home’s voltage is only 90 volts when it should be 115, no since taking the AC apart to see what’s wrong with it, not until the home’s power line voltage is up to spec.
If your car uses a fuel pressure regulator located at the end of the fuel rail, with the engine off, remove the vacuum hose connected to it. Look inside that hose. If there’s any gasoline at all inside that hose, the fuel pressure regulator must be replaced. Another simple test of the regulator is to compare the fuel pressure at idle to what it does when you briefly bump the rpm to 1500-2000. It should go up, maybe 10 psi or so, at least briefly. If it doesn’t, that’s another sign the fuel pressure regulator is faulty. I don’t think the engine compartment fuel pressure regulators are overly expensive to replace, so you might take a flyer and just replace it with a new one, see what happens.