Reading this, it occurs to me that the Enterprise-D is the first one to be shown confirmed with Impulse Engines in both places. Perhaps that is part of how reconnecting the saucer was deemed feasible, since both sections now have reliable fast sub-light maneuverability.
Assuming that any ship older than the Galaxy class can, or cannot, use either section of the ship as a lifeboat, then either the secondary hull of the Constitution and refit version has to use warp drive to escape, or the secondary hull cannot work separated from the saucer and cannot serve as lifeboat. The Ambassador seems an odd situation, since the Impulse engine are in the secondary hull, would the saucer then be unable to flee once separated?
I have two possible fixes for this matter. First, that we simple accept the lack of saucer impulse engines on the Ambassador model as an error and assume that it has either Excelsior or Enterprise-B style impulse engines; in SD the reflection of the bussard scoops on the saucer is intense enough to imagine impulse engines in the area where the reflections turn the saucer red. Secondly, since the Ambassador is the first ship with such a huge saucer, we could assume that the relativistic component the impulse engines on the Galaxy had were not yet invented, and engineers decided that no impulse engine then in existence could move the saucer fast enough, and so did not include them.
I prefer the idea that a ship has either an "auxiliary control" or a "battle bridge" and not both (since both are pretty much the same thing). Two command centers is enough, since the ship could be commanded from engineering. That is to say, at least on the Constitution, movie refit, and Excelsior ships, there apparently is an engine room in the secondary hull near a main reactor, and one in the saucer near the impulse engines (or impulse deflection crystal),so one "bridge" for each section should be enough. As to why the Enterprise-B would have a "battle bridge," my only guess is that someone thought it could separate and reconnect. Maybe it could reconnect itself, but not in any way that would be acceptable unless the ship were in extreme circumstances?
I don't know that I think engineers would get rid of the ability for the secondary hull to work as lifeboat, but, like I said above, it might be that the impulse engines on both sections are part of why this maneuver is more routine on the Galaxy-class. Were the saucer impulse engines ever shown lit up on the Enterprise-D, maybe in the Generations movie? In other words, is there anything onscreen that confirms they are impulse engines like the tech manual would claim?