Here's an update! I got a bag of parts from Karas this morning to see if I could salvage this pen. I received a new nib-and-feed unit, and I got a rollerball conversion kit with the plug removal tool.
I had a theory that maybe the nipple on the original feed was defective and unable to make an airtight seal with my converter, and that might explain the ink burping and very erratic flow. That idea is now shot down. I tested the new feed with two different converters (Schmidt K5 and a Wing Sung squeeze converter), and it behaved exactly the same as the first. The only conclusion I can reach is that the Fountain-K has an inherent design flaw; the feed and ink collector is too small and inadequate to control ink flow reliably.
I think maybe this nib unit wasn't ever designed to work with converters. I think it was meant to be used with tiny, small capacity, cartridge-only pens. Although I didn't try it, I suspect it may be perfectly OK with cartridges. I know some people also remove their converter and fill it, then replace it in the pen, and that's likely to be OK too. Using the pen with a converter and filling in the normal way, however, doesn't work right, and there doesn't seem to be any obvious way to fix it.
Regarding the rollerball conversion... It's pretty straightforward and accepts Pilot G2 refills. (It came with a 0.5mm black refill which I immediately set aside and replaced with a 1.0mm.) I was mildly annoyed by the clicky sounds of the metal tip tapping against the aluminum section as I write, but otherwise it works as expected.
EDIT: And just to dot the final "i" and cross the final "t" on this. . . I swapped back to fountain pen mode and put in a cartridge of Monteverde Black. The new nib was dry and a bit skippy (just like the original was before I adjusted it), and the original nib worked fine, and neither of them had any flow or burping problems. And it does have room for a spare cartridge in the pen body, too.
Edited by tonybelding, 09 January 2017 - 17:07.