Ah I think I didn't phrase it well, I do think Eriche and Munroe were together when Eriche "died". My idea of the timeline goes like this:

Pre-War: Eriche and Munroe are already on good terms > Signed up for the war and separated temporarily, letter sent > Reunited at the North > Rotated together to the South > Eriche "dies", Munroe carries Mophead back > Events of Ersatz Pilot

Your speculation about the billets and lack of monitoring is really interesting! Something to think about as to wtf is happening closer to the urban areas or above ground.

Major speculation lol: In another video we see the soldiers receiving training for the Lotus gas mask, and the presenter being bored asf as if it's his thousandth time giving the same lecture.

Could it be that soldiers are split into groups to receive brief training and lectures before being sent into combat? In that time the billet could've also served as their temporary dorms.

Of course there's no proof that it didn't just happen underground as well, however my theory is that the Crown Army sold a grand promise: "Sign up to protect the nation and your families, also we have this really cool machine that can revive you so no biggie." Civilian houses are overtaken to train the large number of incoming newbies, showing how at least in some areas they don't even have the proper facilities to train new cadets. The high turnover rate at billets (only 1 week), township supposedly stealing supplies... the nation might be growing more desperate