As far as I can tell, "impulse" has not been used with "reactor" in TOS so any reference to "reactor" is likely to be a m/am reactor.
My theory is a hybrid one of yours:
A central m-am reactor "3" (in the engineering section) react m-am fuel whose released energies are pumped through a crystal converter assembly that both stores energy and converts some of it back into more m-am fuel.
The m-am fuel is then split between the central m-am fuel pod and the two m-am nacelles. Each m-am nacelle contains a m-am fuel pod that feeds it's own m-am reactor whose function is simply to consumer m-am fuel to warp space.
This arrangement covers the situations as seen in TOS, IMHO. All 3 reactors are collectively the "warp engines" and depending on the situation, problems can involve one or more of the reactors.
For example, in "Elaan of Troyius", the sabotage of the central m-am reactor and it's crystal converter assembly effectively prevents power from being generated from the system (no crystal power, no energy bypass from the m-am reactors).
Or in "That Which Survives", the runaway central m-am reactor is creating too much m-am fuel and energy that it has to go somewhere because they can't dump it. Etc
"Catspaw"
My theory is a hybrid one of yours:
A central m-am reactor "3" (in the engineering section) react m-am fuel whose released energies are pumped through a crystal converter assembly that both stores energy and converts some of it back into more m-am fuel.
The m-am fuel is then split between the central m-am fuel pod and the two m-am nacelles. Each m-am nacelle contains a m-am fuel pod that feeds it's own m-am reactor whose function is simply to consumer m-am fuel to warp space.
This arrangement covers the situations as seen in TOS, IMHO. All 3 reactors are collectively the "warp engines" and depending on the situation, problems can involve one or more of the reactors.
For example, in "Elaan of Troyius", the sabotage of the central m-am reactor and it's crystal converter assembly effectively prevents power from being generated from the system (no crystal power, no energy bypass from the m-am reactors).
Or in "That Which Survives", the runaway central m-am reactor is creating too much m-am fuel and energy that it has to go somewhere because they can't dump it. Etc

"Catspaw"
DESALLE: Keep it up, Mister Chekov. Channel the entire output of reactors one, two, and three into the relay stations.
"By Any Other Name"
SPOCK: The Enterprise is propelled by matter-anti-matter reactors. The barrier we must traverse is negative energy.
...
SCOTT: I have opened the control valves to the matter-anti-matter nacelles. On your signal, I will flood them with positive energy.
"Elaan of Troyius"
SCOTT: Our shields will hold for a few passes, but without the matter-antimatter reactor, we've no chance. Captain, can you not call Starfleet
"The Day of The Dove"
SPOCK: Alien detected in the engineering section, near reactor number three.
"That Which Survives"
SCOTT: Then you're right, Mister Spock. Watkins must've been murdered. I sent him in to check the matter-antimatter reactor.