Consider: jump nodes require a gravity well in order to stay stable.
I haven't perused the Freespace reference bible lately, but the techroom topic on subspace only mentions the requirement of a massive gravity well for short range intra-system jumps: In other words, that type of subspace travel is only possible because of the gravity well (star).
Unlike inter-system jumps, an in system jump is essentially creating it's own bridge to another point in a star system. Before the jump drive is engaged there wasn't a corridor from point A to B: The drive both allows transit through subspace and creates
the road you travel along, as it were.
Subspace travel between systems utilizes existing corridors; the starting point and destination aren't in our control and the corridor existed before anyone fires up a subspace drive. The gravity well requirement isn't mentioned, and I would posit that it's not a factor.
As for why nodes might collapse on their own: According to the techroom, most subspace corridors are extremely unstable, the ones in use are just those nobody expects to collapse soon (an interesting definition of permanent). Keep in mind this is just based on Terran and Vasudan research into the phenomenon, so if nodes act that randomly without introducing catastrophe into the equation, explaining node collapse due to supernova becomes even more of a leap.
On the topic of subspace, I tend to view subspace travel in terms of wormholes: Inter-system drives allow you to travel through natural
wormholes, and the intra-system drives generate short lived and artificial wormholes, but only inside of a gravity well. Whether the mechanics of a (theoretical) wormhole and subspace travel are in any way similar to each other doesn't really matter, it's just a way of looking at the concept. But it would explain why you can't just fly through subspace to leave wherever you like, as it's very much a tunnel that requires a start and
end point before hand (for in system jumps) or a tunnel that only goes one place anyways (inter-system jumps).