You should be able to remove the nib by putting the nib unit in some hot water for a few minutes and then using some grippy material (like the stuff S B R E Brown uses) and pull the nib & feed out of the nib unit. You may need to wiggle nib to loosen it.
As for ink on the section, I got that on mine with the original nib. I put a little silicone grease on the threads of the nib unit before screwing it and that stopped it.
I have since got a new nib unit from TWSBI, as the collar had cracked and nib kept drying out (great customer service). The new nib is like night and day. Writes beautifully and doesn't dry out or leak!!
Before the new nib unit, is the reason why I thought about the FPR nib. It worked very well, but it requires quite a bit of pressure for flexing, and like you was a bit concerned about the strength of the TWSBI. Besides the new medium TWSBI nib was smoooooth!
The nib worked better in the 530/540/580. May being a larger pen, it didn't feel like it needed as much pressure as with the Classic. I used the TWSBI feed rather than the ebonite. It seemed to work better than the ebonite.
I hope that helps.
Edited by shrey, 29 April 2015 - 18:36.