1st of all, I think you might have your mords wixed. The reducing valve and PRV (pressure reducing valve) are the same thing. Do you mean the TPR (temperature & pressure relief) valve is the one that seeps above 30psi? That would make sense.
2nd, no, I don't see why you should have to drain the system down, just get it to zero pressure. Turning off the supply and briefly opening the TPR should do the job just fine. No need to even remove the tank.
Setting the expansion tank pressure is indeed confusing, but high school physics says it doesn't need to be exact. If it's set slightly lower than system pressure, there will indeed be an immediate influx of water, which will compress the air in the bladder, raising its pressure. Flow will stop when the bladder pressure matches the system pressure. If it's set slightly higher than system pressure, a bit of expansion will occur before any flows into the tank. No big deal either way.
Edit: brainfart. Only use the TPR to release pressure if it's the only option. Safer to open an ordinary valve if one is available.