The expansion tank works best for the circulators if it is before them...this ensures that when they turn on, there's water there that doesn't cavitate (when all's well). But, it should work anywhere on the hot side as long as there's no valve between it and the boiler.
Three reasons the pressure rises that I know of: the expansion tank (ET) is defective or not properly precharged; the autofill valve or the fill valve are leaking or misadjusted; the indirect water heater's heat exchanger has a leak in it, and potable water is leaking into the boiler system from there. I guess you could add an improperly sized ET, but that isn't too common. The pressure relieve valve (it's not a PRV - pressure regulator valve!) on a normal residential boiler is usually 30psi, so whenever it approaches or tries to exceed 30, it's normal for it to release. Since that is an emergency device, after being forced open multiple times, it's best to consider replacing it. It may very well seal, but the water could end up leaving some mineral deposits and mess up its operation, and possibly once you fix things, locking it in a closed position and then things can get dangerous.