I made the measurements again and (as always) techknight is correct about the battery connector, There should also be a diode in series with the 9V battery.

About five days ago, the Portable started squeaking and the display flashing while the floppy drive was connected and/or in use (with battery and power supply connected).

Today I replaced all of the IRF9Z30 with brand new IRF9Z34 MOSFETs, but the situation is still the same. at this point I am pretty sure there is something wrong on the Hybrid or on the traces that go to/from it