I believe most cosmetic things interchange. The 91 has a 5.0 engine and the 94 has a 4.6.
Basically the 94 has a slightly redesigned face (headlights etc.) and I think that's about it apart from the engine.
No the 91 was the first year for the 4.6... Lincoln didnt change the nose until 95 either, 91-94 is the same car
Ah yes you are correct. I get myself mixed up for when they did the new engine deal. There was however still a minor change to the front styling of the car in 93 and went thru til 94.