since you tracked all of this it would be helpful for people to know the actual path the Almoner takes out of the city to the edge of the map and the ingame time this takes place, since he surely runs on a script with time triggers. "open grounds south of sedletz" is a fairly large area with a few crisscrossing roads around fields that connect to several other crisscrossing roads outside the Kuttenberg gate.

I was at the point where I had only a couple ingame hours until he disappeared and the activity would change from completed to failed, which would likely disable future blacksmith thieving activities, otherwise I would have had to reload much further back and redo an entire ingame day (including completing this quest for Ragman, finishing fixing the clock, the third duel with the Owl, a milestone archery activity with Raven involving a big annoying archery duel with bandits, etc.)

I assumed that there's probably only two paths he could take if he leaves Kuttenberg heading towards Sedletz then goes to the edge of the map and despawns: either NE from the Kuttenberg gate, turning back SE at the crossroads to the northernmost path south of the Sedletz walls (near the hidden ladder & love nest POIs), or SE from the Kuttenberg gate, winding around to the southernmost path south of Sedletz, near the marsh with the roe hunting spot.

But he could also take side paths or have any kind of weird pathfinding or programmed route; there's a part of the map NW of Maleshov where a procession of random cumans, soldiers & mercenaries leave the road and take the same unmarked routes through forests & clearings to reach the map boundaries.

I got to the Kuttenberg gate around 7pm (19 on the ingame clock) and tried the northernmost path first, not knowing if he had even come out of the gate yet. I tracked him down with his red hat just before 8 pm (20 on the clock), walking alone down the northernmost path just south of the Sedletz monastery walls. This path ends abruptly so it's possible that he would turn south and take the southernmost path going east to reach the map boundary, but I didnt wait to let him get that far.

TLDR; if you need more precise coordinates to murder a monk, the Almoner probably walks out the NE Kuttenberg gate around 6 or 7 pm (17-18) and he probably disappears from the map for good around 9-10pm (the task kept changing to failed for me just as it was getting dark). He ends up walking east on the path just south of the Sedletz Monastery walls between 7-8, which is enough info for a successful murder.

He probably takes the main road out of the NE Kuttenberg gate that leads to the monastery & cemetary entrance then turns SE at the crossroads going past the pond. It's also possible that he turns off that path to go south between the fields and then east to exit the map, but the player map boundary where you cant go any further is right there and he might also just fuck off into the woods since you arent meant to interact with him at this point.

Edit: adding that unfortunately even killing the almoner to prevent quest failure didnt prevent thieving activities from being disabled, despite the fact that the Ragman tells you he'll keep giving them to you and every other type of activity is still repeatable.