Dave,
nice shape - i've not yet tried to go beyond the straight or naturally curved (to meet the various fittings).
As far as taking them apart I have had to to do this too often, and the twist mechanism is the hardest and often ends up broken.
I have had some success as follows.....
get a ring spanner (or metal with appropriate hole) so that it slides over the mechanism and rests on the lower wood. keep the refill in the mechanism, grip it in a metalworker vice (protect as much as possible). use another tool (e.g screwdriver) as a fulcrum on the end of the vice and lever the spanner - if you're lucky the mechanism will come out - it definitely will if you fully tighten the vice and sacrifice the mechanism to get the wood back.
If you assemble and discover the nib is to small /too big and you want to return to the lathe, use a 6mm drift, holding the wood in your hand - normally 4 knocks and it's out.
good luck!
dave