As far as I understand from Grimm's treatment of German unterstellen "suppose", it started literally as to find a place to stand under (earliest evidence from C8), thus to park horses, to stay over (over the night) and from there took on a spiritual meaning as to seek shelter, confide in higher powers, confess to a believe, to believe, to suppose (now chiefly bi-transitive and negatively connotated in German, to believe somebody stupid). Indeed a Unterstand is still simply something to stand under, a stand, an overhang and similar (chiefly if it rains); a stay in English has similar meaings, a short stint seems comparable and to stall basically mean "stop"; The stall, Ger Stallung (animal enclosure) rather derives from a sense "firm", though.
Now I have no proof that this directly parallels the English situation. I don't suppose it's the complete picture as suppose on the one hand looks mighty similar, but German verstehen "to understand" needs to be compared as well. ver- often has a connotation of change (verhandeln "negotiate", Handel "trade"; versichern, Versicherung "insurance"), though four or five other ones, too.
This inspires the following comparison, though under does not directly compare to inter (if from PIE *h2en- vs *h1en- respectively, which is not entirely certain). Yet I'd suppose that inter- might give the necessary insight, which etymonline and several answers relying on it agree with, glossing "among".
@jlawler's allusion to behind given in the other thread cannot be dismissed either, if stand behind basically means support (also in German at least).
For the previous too paragraphs compare Ger einstehen, with a much stronger connotation than to stand in [for somebody]--it rather means support in general, in certain contexts.
This will have to suffice for an answer about an uncertain etymology, before I go off on a tangent about stout beer, shortbread, Germanic vowel alternation i, a, u (implying under was used as a preverb with perfective aspect), stood vs stutz, stütz, that would be ultimately undecidable because of the principle of explosion.