> Not sure about the first syllable of "poplitibus"

Lewis and Short doesn't mark that o short or long. This also occurred with vexo, which I assumed was because the first syllable of anything starting with vex- is necessarily long (because the 'x' is two non-liquid consonants).

In the case of poples, 'l' is a liquid and a short vowel could be revealed by the syllabification po-ples (or po-pli-), but I guess this is never attested? This verse can't answer the question because the syllable is allowed to be long.