"So, where is B deck? Immediately beneath the bridge? Why would that be a good place to sabotage the impulse engines from? If it's some place closer to the impulse engines, then why would it be called "B deck"?"
If "H deck" refers to the hangar deck, than "B deck" could stand for Berth deck
and there are good reasons for that: It's a naval term from the age of wooden sailing vessels, "a deck next below the gun deck, where the hammocks of the crew were swung."
According to the official description of the saucer decks, "the decks 4, 5, and 6 are primarily crew quarters, with some provisions for passenger quarters." The senior officers quarters are on deck 5 while the other crew members and junior officers have their quarters on deck 6 (the "Lower Decks" to speak with TNG) which would be the "Berth deck" or simply "B deck".
The impulse engines are at the stern of deck 6 or "B deck".
You may trouble me for the salt, as I assume I have no further need for it.