I do think that the inclusion of materials outside of the aired material does needlessly confuses things.
For example, the only time "fusion" is mentioned in aired TOS is in regards to overloading the impulse engines on the Constellation in "The Doomsday Machine". It is not mentioned with the warp engines or matter-antimatter reactors.
TOS does confirm that when matter and antimatter meet they "cancel each other out, violently" ("The Alternative Factor").
The "matter-antimatter integrator" is spoken of in "That Which Survives". An "integrator" is someone or thing that "integrates". "Integrates"
according to the dictionary is
1: to form, coordinate, or blend into a functioning or unified whole : unite
2: to find the integral of (as a function or equation)
3a : to unite with something else
3b : to incorporate into a larger unit
4a : to end the segregation of and bring into equal membership in society or an organization
4b : desegregate <integrate school districts>
So, "blend, unite or desegregate" matter and antimatter using an integrator and you would get a matter-antimatter reaction (or explosion) which is consistent with how TOS presented it and without the extra "nuclear" connotation that "fusion" brings along with it.
It brings back the simpler, explanation:
- Matter and antimatter are annihilated (when integrated ) and the product energy is sent to the crystals.
also is just simply "to mix together". We know it was used in reference to mixing up matter-antimatter in "The Naked Time". TMP adds a bit of a wrinkle as it is used here:
SCOTT: Intermix set. Bridge, impulse power at your discretion.
KIRK: Impulse power, Mister Sulu. Ahead, warp point five. ...Departure angle on viewer.
Which is interesting since it could be construed that from only the dialogue that intermix has something to do with the impulse engine reactors. In this case it could, depending on your viewpoint. On the other hand, he calls for "warp point five" instead of "space normal speed point five" or some other non-warp related speed. This could be interpreted as the impulse engines can accept matter-antimatter power and/or capable of a (sub)-warp field.
There are interesting bits from "Pen Pals" about dilithium turning heat into mechanical energy. In Voyager they have some episodes describing dilithium as fuel.