Ok, I've done some more research and found a good review article by Sara Seager:
Here is my understanding:
The TESS (transiting exoplanet surveying satellite) will discover a large number of exoplanets using the transiting technique (like kepler). Some of these may be good candidates for observation with the James Webb telescope. If we get really really really lucky we might find enough exoplanets that happen to transit their suns in a convenient way for us to image and do atmospheric spectroscopy. Its unlikely that enough/any data will be forthcoming to get any reasonable probability that biosignature gases are present.
Many decades from now:
If this doesn't work (and it probably wont) then some form of direct imaging is required. Two technologies are discussed:
Internal coronography - special optics in the space telescope which blank out the light from the star but not the planet.
Starshade designs - a shade is flown tens of thousands of km away from a telescope and positioned very accurately to block the light from the star but not the planet.
If this type of thing sounds difficult, expensive and time consuming its because it is.
Build a massive telescope that can directly image an exoplanet. Not going to happen any time soon.
The issue with this type of work is that a large number of sample planets are needed to be able to overcome the large number of false positives that are undoubtedly going to be discovered.
So to answer my question, yes the JWST can perform exoplanet spectroscopy but its unlikely that it will give us any definitive answers. Optical interferometry will not be needed and the above descriptions of future technology should give us the answers we need.