Using the (parasitic) reverse diodes of the MOSFETs for protection purposes will affect the precision of the sensitive ranges, due to their leakage currents. These lower ranges resolve nA, maybe pA currents.
Especially, if you apply a reverse current, in the "negative" direction of the DMMs input jacks, these 50mV burden voltage already create relevant forward currents in these MOSFET diodes, and if you combine two of such MOSFETs in parallel, this would be the case for either direction.
More common precision current protection schemes use either the low leakage of the B-E diode of a bipolar transistor, like in the HP3458A, which includes power dissipation protection by software, or they use at least two diodes of a rectifier bridge in series, combined with an OpAmp to reduce these diode leakage currents to near zero, like in most of HPs 6 1/2 digits DMMs.
Daves last circuit with one shunt for each decade can be used in DMMs with electronic gain calibration only, as the shunts for the different ranges have the sequence 0.01, 0.11, 1.11, 11.11 and 111.11, not a decade sequence.
Therefore this can't be used for an external front end, like the µcurrent.
Frank