Why is the decarboxylation of 2-hydroxybenzoic acid (and related compounds) done 'under nitrogen'?
[&hellip;] it seems even more important to get the right mixture of gases around it at 122℃


No, it's only important to keep the oxygen away while you are heating the sample.

Have a look at the structures: the thermal decarboxylation of the THCA isn't different from the decarboxylation of salicylic acid.

But with the carboxyl group or without, (upon heating,) phenols are easily oxidized by air.

In principle, the sample could be heated under vacuum too. In this case, you might want to use a cooling trap to collect volatile material. But usually, one does that anyway to protect the vacuum pump.