The within reach wouldn't be a problem, because segments are only a few tiles apart. And also we can add teh tag SENSE_CREATURE_CLASS:themself, giving them always eachothers location.
The interactions: you named two concerns that I share.
1. The interactions are used to connect the segments. And to be honest, those are unreliable. Maybe an option is that the creatuere's head spawn items or objects that have the same graphics / image as the segments. So when the head moves, the image of a segment stays behind for a few ticks, giving the segments time to reconnect.
2. The interaction to spawn the segments. What I was thinking, is to make te worm an ambusher. Meaning that when the worm gets visible, it is in combat, giving it a usage reason for the spawn-interactions. Also, that hostility can help keeping the segmetns attached.
So, what you said.
Also, another idea, If I make the worm SENSE_CREATURE_CLASS:GENERAL_POISON, than it has always enemies in sight, keeping it always in combat, and giving it always reasons for usage_hint ATTACK
I can give the attach-interaction like all the possible usages hints, so that in any case, the creature will try to do it.
I would prever to not use DFhack, because that makes the mod harder to 'sell'. So, i'll keep on thinking on it.