But… belief is irrelevant. You need to examine the canon and derive a hypothesis that fits the facts. There is M/AMR in the nacelles and maybe elsewhere. Multiple M/AMR reactors, as opposed to the central one in TMP+, would best explain conflicting reports on the subject. Here's a possible outline:
1) In TOS they're extremely dangerous and must be kept in the nacelles, away from the crew, so the reactors are pretty much restricted to warp power generation. Later, engineers start experimenting with smaller reactors in the main body.
2) In TMP they move everything to the main body, in order to gain more power for other ship systems, but still with mandatory radiation suits.
3) Later, the shielding improves and no protective suits are needed.