Hmmm, minimum machining, simple tools and low force on the trigger?
What would you think about a toggle-link lock? It can be constructed simply from bar stock with mostly just drilling and very little machining, no sear faces that have to be matched and the force to release is also very low on the trigger? Its not a kind of lock that is commonly used, the only application i have seen so far of a toggle link assembly in commercial crossbows was in the swiss twinbow...
something like this:
just five pieces of bar stock with holes drilled and a trigger to push up on the central pivot point.
The thing to watch out for in this kind of lock would be to space the distance of the bars pivit points such that in "spanned" position the central pivot is very slightly below centerline so its pushing a little downwards when force is applied. Thus the main draw force is directed backwards and only a little amount is pushing downwards making for an easy release. You can add a small magnet (blue) to help keeping it in position initially but once force is applied its essentially self locking. If you need more distance between the "claws" just use a pieceof thicker Stock or double up on the first horizontal linkage bar. Exact matching of the pivot holes could be easily achieved just by stacking up the pieces and drilling them at once.
If you want even more controlled release one could replace the simple "push-up" trigger part with some king of spring driven "hammer" which has to be pre-cocked andis released by a trigger thus converting it into a set-trigger.
mechanically the toggle link makes for a very strong lock and because of the force distribution only little force is requird to release it.
Maybe there are better options but that is what I came up with when looking at the OPs initial requirements.