I participated in the round (played as lurker, I wasn’t harmed by the accused directly) and witnessed Nathan using the blue flame sentry in two occurrences:
Occurrence one: the synth and another marine were near secondary comms, north (FOB was south LZ). The accused put his flamer sentry in the grass south of the comms, facing also south. He and the marine were standing in the sentry zone of fire, in order (from my perspective) to bait backliners to attack them so the backliners would be ambushed by the hidden sentry. The accused stated in discord that he merely wanted to secure second comms for the nuke, but again, the sentry wasn’t covering the comms, rather than the backline (also comms weren’t repaired nor caded). And the placement of the sentry in the grass also gives away the intent to catch xenos by surprise, rather than to discourage them from attacking.
Occurrence two: mentioned by the OP. Xenos were besieging the second comms later in the round, and were resting behind a wall. The accused (and I suppose some force of marines) flanked them, the accused placed the sentry the way it immediately started to shooting resting xenos. Again, in discord the accused stated that he merely was securing the perimeter to protect marines, but again, I don’t believe this is true, as it was a flanking maneuver.
In any way, I agree with the OP that placing sentries in combat seems to me against the spirit of synth guidelines, even though it’s not stated directly.
Synthetics are forbidden from firing any weapons, including direct fire emplacements (M56Ds) and indirect weaponry, such as CAS, the M402 Mortar, and OBs. However, they are not opposed to other humans harming/killing others provided it follows Marine Law. If there is a threat within the bounds of Marine Law, they can physically defend themselves or other innocents by subduing the threat non-lethally.
It seems to me that the use of sentries in active combat (and in backline) falls under the use of indirect weaponry. I can understand how a synth moves the sentry in the FOB to a better position, outside of an active siege. But deliberately carrying a sentry for use in active combat strikes me as a violation of the rules. Hunting the backline with a sentry as a synth also seems like a clear violation of the rules to me.
Nothing personal to InsaneRed too, but it feels like an obvious attempt to circumvent the rules of synth combat to get an advantage in combat as a synth.
I want to mention this too, but this is the first time in years I see a synth deliberately carrying a sentry. I played engineer a lot of time and from what I have seen, if a synth finds a sentry, they usually give it away to the closest engineer immediately. For me, the fact the accused was carrying the sentry for the huge portion of the round clearly shows his intent to use it in combat.