I tend to agree that MJ designed the action to be going on the nacelles. However, certain episodes (such as That Which Survives) clearly point to a single area of reaction between matter and antimatter.
In a MJ-nacelle style setup though, I believe that the ship could enter Warp even with the lithium crystal circuits all blown. However, since things like inertial dampeners and deflectors would be unpowered at this point, you really wouldn't want to!
@Mytran - Yep which makes things a little complicated. I was thinking that the ship could've been originally nacelle powered in the first season or two and then switched over to a central core in the 3rd season but "The Savage Curtain" nixed that.
So a solution that seems to work is that there are three m/am reactor cores and they can all be referred to as the m/am reactor (system) or individually (central) or the pair for the nacelles
@T'Girl and Psion - Based on "Mudd's Women" and "Elaan of Troyius" it would seem that the lithium/dilithium converter assembly has or can by rigged with a bypass circuit. In "Mudd's Women" they were burned out when the engines superheated and in "Elaan" the assembly was fused. "The Wrath of Khan" does show a successfully rigged bypass of the main energisers though. In anycase, it does sound like with a successful bypass and on m/am power only (no dilithium) a very low warp speed is possible via the warp engines or impulse engines pushed to FTL, IMHO.