West Germanic infinitive suffix
I noticed that both German -en and Yiddish ־ן (-n) don't have adequate etymologies. Both should go to Old High German and back to Proto-Germanic, but looking the more complete etymology at Middle Dutch -en only confused me, because these infinitive terminations are used for every kind of stem. Can you clarify the etymological history here?
The Middle Dutch etymology really says all that can be said as far as I can see. The infinitive suffix -en in German doesn't have a single definitive etymology. Originally, the infinitive suffix was just -ną in Proto-Germanic. Various vowels would appear before that suffix depending on the type of verb, giving -aną (strong verbs), -janą (class 1 weak verbs), -ōną (class 2 weak verbs), -āną (class 3 weak denominatives), -janą (class 3 weak statives), -naną (class 4 weak). Most of them were still distinct in OHG: -an, -(i)en, -on, -en, -non respectively. But because unstressed vowels became schwa in the transition to MHG, all of the suffixes fell together into a single common -en and were no longer distinguished. So the answer to "what is the origin of the German infinitive suffix -en" is "all of them!".
!אוי וויי Do PG appendix pages exist for all those suffixes? And was the merger complete (I guess as -en) in MHG?
It was probably mostly complete. It's possible that -jan was still distinct after -r-, as it was in OHG. Look at swerien or werien. I think we do have appendix pages for them: *-anan, *-janan, *-ānan, *-nanan. The original suffix -ną wasn't productive anymore and only occurred in a few single-syllable relic verbs.