For me, there is something else to consider. Impulse is synomous in Star Trek with nuclear-powered engines. The S.S. Valiant and the Romulan Bird-of-Prey were both described as having impulse, yet were described or shown to be interstellar ships. So, these ships in my interpretation had a primitive warp drive that involved a nuclear reaction of some kind.
I agree that Enterprise was an attempt at retroactive continuity.
That's right, the "BOT" Warbird and "WNMHGB" Valiant were impulse-powered yet interstellar. Good find on the SS Mariposa. Personally, I don't think they would have had crystals for their fusion power systems but under the ENT-continuity they probably would've had a M-AM Warp 2 engine with a dilithium matrix.
Although I wonder if ENT overwrote "Up The Long Ladder"'s history since the dialogue placed the WW3 recovery into the early 22nd century and the launch of the SS Mariposa occurred under the European Hegemony which lasted till 2190 and was considered the "first stirrings of a world government"...