I don't think Oswald had a "Mexico plan" beforehand. Once he got out of the building, what does he do? Does he head toward Oak Cliff right away? Nope. He heads east on Elm, deeper into the downtown area (and completely away from Oak Cliff). This one fact alone tells me that he had no escape plan; only to indeed escape.
I believe that once Oswald made it to Whaley's cab, the idea of going to retrieve his revolver appealed to him. Once he had his revolver, does he head to the theater and/or south into Oak Cliff? Nope. He stands at a bus stop outside the rooming house, very near the corner of Beckley and Zangs. This particular bus stop would allow him to catch a bus going north, back toward the downtown area, NOT south into Oak Cliff or Mexico. I believe he wanted to board the first bus to get him the hell out of the area.
Then, after standing near that very busy intersection (Beckley & Zang is an incredibly busy intersection), he thought better of it. He was on full display standing out there at that particular bus stop. He left his rifle behind. He was missing from the building in which he was supposed to be working. Has his face been on the news already? He's been on foot, bus and taxi for a half hour. He doesn't know what has (or has not) transpired during those thirty minutes. Obviously we know his face wasn't on the news but he doesn't know that. In my opinion, he shows the same paranoia when he doesn't pay for a ticket at the theater. He doesn't want Julia Postal to see his face. Has his face been on the news? What if she immediately recognizes him? Or, perhaps she'll see his face on the news while he's hiding out inside the theater and recognize that face as belonging to a guy she sold a ticket to earlier. By the way, it is this same paranoia (has my face been on the news already?) which I believe causes him to switch directions as he's walking along Tenth Street. He doesn't want the approaching officer (Tippit) to see his face. In my opinion, he also avoids walking past a law enforcement vehicle (Deputy Sheriff Unit #109) moments before the Tippit encounter as he's walking toward the library and/or bus stop outside the library.
My opinion is that the Jefferson Branch Library and/or the bus stop right outside that library was his goal once he decided to bail on the idea of standing at the very busy intersection of Beckley & Zangs. If one is standing outside the rooming house at 1026 N. Beckley and the goal is now the library and/or the bus stop outside that library, walking east on Tenth at Lansing Street (exactly where Tenth Street makes it's drastic curve) is the most direct route, along with another route which would have him walking down Marsalis for most of the way. But he doesn't want to walk down Marsalis, a very busy street.
I guess my point in all of this is that, again, in my opinion, maybe making a run for Mexico is only something which occurred to him once he decided to bail on the bus stop outside the rooming house. Obviously there is no way to know for sure. Only Oswald himself knows the answer. When it comes to trying to determine Oswald's thoughts and movements after the assassination, all any of us can do is speculate. However, some of us speculate based on what we know while others (like the "Oswald never got on the bus" crowd) discard the known facts and speculate with no basis or foundation.
As for the sixth floor...
As I've always seen it play out in my head, Oswald has the brown shirt tied around his waist, as people typically do when they're originally wearing two shirts but it becomes too warm for both. I believe Oswald is still contemplating which end of the sixth floor to shoot from and is over at the southwest end of the sixth floor when Bonnie Ray Williams unexpectedly arrives up on the floor, surprising him. Oswald is standing near the southwest corner as Williams is eating his chicken on the bone sandwich (what's up with that, by the way?) over near the southeast corner. The south face of the Depository is captured in aftermath photos and the far west sixth floor set of windows are open. Arnold Rowland sees a guy with a rifle standing back from that west end window and says the guy is wearing a light-colored shirt opened at the neck. Perhaps Oswald still has the brown shirt (CE-150) tied around his waist.
Williams finally leaves for the fifth floor and Oswald decides that the southeast corner window is the easier shot (which it certainly was, study sniper fire and lead). Whether he still has the brown shirt tied around his waist or it's simply lying on the floor in the sniper's nest or atop one of the boxes doesn't matter. After firing the shots, he grabs the brown shirt and uses it to wipe the rifle as he makes his way across the floor. This is why a tuft of fibers matching that shirt were found in the crevice of the rifle between the metal butt plate and the wooden stock. He reaches the stairs, stashes/hides the rifle haphazardly and proceeds to put on the brown shirt as he's going down the stairs. He probably has the shirt on by the time he reaches the fifth floor.
