So finally I managed to push the bar/pin out, and you guys are right, it's a pin and collar setup. Here's the bar/pin (and collars) with Citizen NY0040 spring bar for comparison purpose:
Because I hadn't been able to find a fitting pin to push out the watch's bar/pin, I used a 1.4mm screwdriver. Bad idea don't copy! Why, because:
1/ It has sharp edges and might damage the collar (fortunately I didn't).
2/ It fits the bracelet holes but not the case "lugs". Someone who doesn't know might push too hard and damage something when it stops.
So at the end I used a straightened and rounded paperclip to push the bar/pin out. It's best IMO to use a pin that's smaller in diameter than the collar (about <1mm) so the collar can slip effortlessly onto the pin.
Now for an important part: The bar has only one collar (the other end is a raised-larger diameter of the bar/pin itself) so you must push from the collar side else the bar/pin won't budge. The collar side is at the watch's left side on *MY* watch (I don't know the repair history of this watch).
Now that the bracelet is free off the watch case, I can comment about my observation of the construction of this arrangement. The bracelet-case fixing definitely doesn't look robust! Reasons:
1/ The "lugs" are tubes welded to the case:
2/ The bracelet endlinks are sheet metal welded to the bracelet and then folded to make a tube:
Maybe tomorrow I'll add the dimensions of the bar/pin and collar when I have access to a caliper.
BTW how to remove the bracelet links (those damages weren't made by me)?