Well, I did test further, but the outcome is not thrilling (although it was fun).
As suggested by mhguda (thanks), I did pull the nib out slightly which does give a margin more flex, not a great deal though, and in this position the feeder tends to move out of place (misaligned with the nib).
So I went on to test a few other #6 nibs, making also use of some spare ebonite feeds received with other Ranga pens
(thank you Ranga, I love the extra feeder bonuses, even more than the freebie pens...).
I first tried a Stipula nib, both a steel M and a 14k 1.1 stub, unfortunately both of them are much thinner than the Kanwrite, so they slip down the section too easily and the nib cannot be placed in the correct position. Even positioned loosely they do write and it's a big difference with the Kanwrite, but they are not firm enough into the section to be used.
Next test was with a Delta 18k Millennium nib F.
Absolutely wonderful nib, but same problem again the section is far too wide to hold this nib properly.
Further test with a Bexley 18K M nib.
Lovely nib, but far too loose again in this Ranga.
I then tested a Jowo steel nib B (disassembled a Jowo nib group by pulling out nib and feed from the collar, it's really very easy to do).
Slightly better but still too loose, the Jowo is ever so smooth though.
Last test I took out a Bock steel nib M from another Ranga 8B.
It came closer than all the other tests above, but still no sigar.
There is still some slack.
I made an inverse test, I tried fitting the Kanwrite in the other 8B. Too tight!
(I also tried fitting the Stipula, Delta, Bexley and Jowo in it, slightly too loose!)
In practice the Kanwrite seems to be made from a very thick sheet of steel, so the section of my Ranga 8B green-olive ripples is machined to hold this combo and will not take other nibs I own (i.e. they all fit too loose), despite all nibs looked as they would match well with the feeder (some testing with ink confirmed they would, no hard starts and very nice flow).
I tested both the feeder that came with the Kanwrite, and also other spare Ranga feeders I have (about 6 different ones), but the result does not change, all feeders looked alike in width and made no difference.
The interesting lesson learned is that the tested nibs are not the same thickness,
The Kanwrite is much thicker than the others, then comes the Bock, then the Jowo, then possibly the Bexley, the Delta and the Stipula (even the steel one) are even thinner than the other nibs.
I will probably keep this ED Ranga with the Kanwrite nib (unless I can ask Mr. Kandan to send me a slightly larger feed...),
I will now probably play around with my other ED Rangas to see whether I can swop in some other nibs...
(I have a Stipula titanium nib somewhere that I did not test... )