The central locking vacuum pump (in a W124) is controlled by the front doors- and trunk locks. The vacuum pods in these doors and rear panel are partial vacuum and partial electric controlled. The electric wires from these pods go to the vacuum pump under the rear seat (at the right).
Alarm systems are normally connected to the wires of the front (preferable left) door(s).
My guess is that this (un)locking by the trunk lock is caused by the trunk's vacuum pod or connecting plug/wires.
As far as your problem with the left front door lock goes: order a new lock at the dealer (by chassis- or VIN#). You will get a lock that fits your original key and it will not cost that much.
You can take apart the lock as well to clean it, but that is only a temporary solution...
with a minor 600+k