Keeping in mind, too, that the customer is not going to be "assembling" their pen back when they change their refills. They are just going to be screwing it together, with only the exterior, finished parts exposed, unlike us when we assemble them. I'll try the school paste.
To answer the question about the ink replacement, they actually replace it often enough to be an issue. I replace my refills with gel rollerballs which write nicely but run out sooner. Especially when someone loves their pen and they begin to use it exclusively, it runs out after a couple of months, if not sooner. My father in law loves his pen and writes with it exclusively, and he has to buy his refills in pairs because he blows through them so fast. This has also led to my previous threads about smooth writing gel rollerball or pressurized ballpoint refills that fit the Parker style that last longer than Parker brand gel rollerballs, which is what I'm using now. I haven't found a good solution there yet.
But I digress. I looked up some old posts about Baron caps coming undone and that led me to the idea of an o-ring, that would sort of "lock" down the nib section to the twist mechanism in the Sierra. It would have to be one small, thin o-ring, so I think a Home Depot trip is in my near future.