Not necessarily. The neutral/ground of each enclosure will not "itself" create a low impedance parallel path between enclosures.
If the trough and nipples are metal and bonded per code then they most certainly will create an additional low impedance path.
If there is no other low impedance path between enclosures, and the GEC taps are removed because the GEC is now connected in the trough, the only low impedance path is the neutral feeder between each panel and the trough. The trough becomes a "true" single point bond, trough connection notwithstanding.
Nope. The nipples and trough are another parallel path, unless you use PVC parts somewhere along the line.
This is critical. I'm showing total current on the neutral and GEC. The color I used is representative of current flow as combined/cancelled as the case may be. Note the 90A imbalance of current arriving on panel 2 is opposite phase to the 60A imbalance on panel 1. We are taught that when this happens, the net is 30. And the net is 30....back to the transformer. All is well. What I"m showing is that while the 90/60 gives us 30 back to the transformer, there is 50 on the GEC taps between the neutrals.
The imbalance between phases in panel 1 and panel 2 is reconciled across the GEC taps!!!!! A perfect balance back to the transformer can result in substantial current on the GEC taps between the neutrals. For example, with my previous assumptions, 100 on panel 1 and 100 on panel 2 (split phase to panel 1) will result in a net 0 back to transformer. Hang a current meter on the GEC tap and it will read 67A!!!!
Assuming negligible difference in impedance between the two paths (and pretending that you
are using PVC between the discos and trough), here is a corrected version of your diagram.
Now take away the 60A load on the left. Note that the magnitude of the problem hasn't changed a bit. I don't believe that phase balancing between the two discos makes the issue worse.
The neutral (and trough) are likely to have a slightly lower impedance than the GEC tap route, so if anything, slightly more current will flow the 'correct' way, although the difference may be negligible.