I'm personally of the opinion that a keychain pen should be as small as possible, when closed, so the original Inka Pen wins on this point. Also for situations where you just need to write a short note or sign something, the short part of the pen is perfectly suitable. However, I agree that adding the extension tube makes too long and is too fiddly. This is all a bit academic though, since the original Inka pen is no longer manufactured.
The NiteIze Inka pen just seems bulkier that it needs to be, has an overly large gate clip and is longer when closed than it is when open, which is a little bit of a poor design, for a keychain pen. Plus on my version, one of the caps has started to crack, due to it's overly tight fit.
A while ago, I had a play around with the basic design of the original Inka pen, to see if it could be made a little more functional, in it's current size.
I've made two variations on the same basic design, that work around the diameter and gauge of steel tubing that I could find at the time. I ended up making a hybrid of the original Inka pen and the Ohto Petit-B (both shown in the lower photos).
The blue plastic parts are 3D printed and the refill is a D1 mini, the same as both the Inka pens.
Here are some comparison photos: