I commented my feelings on the 'was this voluntary or ordered' in the main 6.4 thread, but while it wouldn't surprise me to have a retcon clarify that Emet was not accurate in his statement that the first two sacrifices to Zodiark were taken voluntarily (the third decidedly would not), I still don't think it was an outright draft conscription. I mean, you could say not dodging your draft is willing offering yourself up. But I'm not that cynical on Emet's Rose Tinted Glasses.
But societal peer pressure to not be selfish and offer yourself up to the star instead of clinging to life for a larger goal was already how their society functioned. What we don't have any answers to and what I think Erichtonios's statement hints at is that we aren't told if the Convocation's call for people to offer themselves up to Zodiark was a blanket request to everyone (well, except they were only going to allow one member of their ruling council to volunteer and not for any old position but that of the heart, and I would have to recheck the lines to know if the choice to go with Themis instead of Gaia was because she rescinded her nomination or was outvoted in favor of Themis) or there were people encouraged more strongly than others to sacrifice/not to.
That a Convocation's family member was supposed to be on the altar could suggest many things- that Lahabrea expected his son to be more nobly altruistic than other citizens, or the political optics that while he personally wasn't the one sacrificing himself for the plan he created, that Lahabrea was willing to do this with his son was an effective tool in convincing the average citizen of the validity of his theory and that they had no excuse to hold out. Hythlodeaus has high government bureau position in common with Erichtonios so maybe they were under more expectation than non-government employees- but we don't actually ever meet a regular Amaurotine citizen by name, just the nameless shades in fake Amaurot.
And it's Eleventh Hour Desperation because the rest of the planet is gone while the Convocation debated what if anything to do, so they're in panic mode and what else is there to do but die so might as well volunteer for the suicide mission that maybe has a chance of success.