I just came to the realization that the mich inflator produced higher pressure when read by the china gauges was by design. Air heats up when compressed. The mich was compensating for that. This is why, once cooled down, the pressure in the play wheel stayed static and at the intended level.
Here's an additional thought in dealing with tyres and old wheels. Even if it's impractical to do DIY tyre mounting because of potential paint damages, there remains an option to dismount an old tyre by physically cutting it apart. Then the bead seat could be prepared/repaired before giving the wheel to the shop for mounting. This will reduce chance of leaking on old wheels with corrosion. Most small shops will be happy with this kind of arrangement.