If you want to use the stub nib and have another TWSBI Eco, you can just replace the nib, so that shouldn't be a problem.
However, your issue with the piston mechanism is worrisome.
It is either something that could be fixed by adding some lubricant as others suggested or by forcibly taking the pen apart.
I've had a similar issue: I used KWZ IG Green Gold in my TWSBI Eco and the ink basically destroyed the barrel. (This is a known issue with older versions of the KWZ inks, but they accidentally sent me this instead of the new version where this problem has been eliminated.)
Here's what I did:
I forcibly screwed off the knob using the wrench that was supplied with the pen. The piston itself was stuck in the barrel however, so I used a wire from the nib end to push out the piston and the seal. Fortunately, most of the piston mechanism survived. Unfortunately, the piston seal was damaged while I pushed it out. So I needed a new barrel (because the KWZ ink destroyed it) and a new piston seal (because I damaged it). The guys at KWZ however were extremely helpful and provided me with both (and a new version of the ink which is no longer susceptible to cause such damage), free of charge. I managed to put it together and added a little silicone grease to the new piston seal, and it works like a charm ever since.
I do not, however recommend using brute force to you unless you feel confident that you can take it apart without damaging it.