Ah yes, that was mine I think.
Not a silly question, undoubtedly the network could probably be improved with a bit more effort.
Regarding the limited trim range, no it doesn't cover the full worst case offset range of the LMC622, however in practice it was fine for the 'typical' offset of the sample I used (I had measure its offset at the start anyway). If neccessary the component values could be tweaked for greater span but I was shooting for maximum null resolution on the pot. If I was going for volume production I would have expanded the range but would probably screen the LMC662s too, as the ones at the offset limits might possibly be less than ideal in other parameters too (input current etc.). Yes, there probably is an element of offset tracking between the two opamps in the package too (not tested).
In practice the offset nulling tracks the opamp offset pretty well as the supply voltage declines. I haven't found any need adjust offset trim. My unit is still on it's original battery and without any intermediate trimming, is still zero to within the 0.1mV digit on the 200mV range of a normal handheld DMM. There don't appear to be any noticable temperature drift effects at this resolution over normal ambient.
I did consider adding a 5V micropower LDO, which would remove pretty much any battery voltage related offset drift, but I haven't got around to it and given the stability over the last two years, probably won't bother now (maybe a lucky opamp sample).
Looking at your revised circuit....
- The diode is another active component with it's own TC, hard to know how things would track or not. Also the diode would be operating at a very low current (less than approx 30uA on a full battery), where its forward voltage and stability might not be that predictable, sample to sample (hard to say off hand).
- Yes, the input to the opamp will be seeing a much higher source impedance ~165k vs 7.5R (once the closest decoupling capacitors are taken into account), again hard to predict what effect this might, or might not, have without prototyping it. Maybe just a little bit noisier, extra capacitors would probably be unnecessary and might add stray leakage paths. The input reistance of the LMC662 is so high that it probably wouldn't make any difference in practice.
- More components, not an issue if it's 'better'.
As I mentioned above, adding supply regulation is probably the most effective way of removing any offset trim issues, at the expense of slightly less battery life and maximum current measurement.
Why not have a play with offset compensation networks for yourself - LMC662s (or another generic cmos opamp) are really cheap and you don't need the 1G feedback resistor if you are only interested in offset compensation experiments.