Tom,
This will have to be brief. For one thing, I have to be somewhere shortly, and for another, I don't have time to review the testimony the way I need to in order to give you the kind of answer your questions deserve.
Briefly, Mullin used the 1916 map, but prepared a revision in 1949 (See my article on the O. K. Corral in the current WWHA JOURNAL).
Coleman said that Ike was last as they crossed Allen and entered the O. K. Corral. What we don't have clear evidence on is the conversation and movements that took place among the cowboys once they were inside the corral. Standing together as they were when Sills saw them leaves us with no insight of how they proceeded through the corral on the part of Sills, Coleman, or Allen. Ike could easily have walked with Frank toward Fremont while the Billys lingered before proceeding to the vacant lot.
It is a theory, but the combination of events suggests that the vacant lot was the place they agreed to meet. That fits whether they were up to mischief or on their way to the West End Corral for Ike's wagon.
If I thought that Billy did in fact tie his horse up in front of Bauer's before going to Fly's then going through to Fremont is the argument I would make, although it would pose a problem too. If Billy was urging Ike to get out of town, as Claiborne also testify, why would he tie up a horse that he would have to go back for before proceeding to the West End Corral? If they were really leaving town, wouldn't it make more sense for him to keep the horse with him?
I need to review what I have on the "back route." We had a discussion on the board which dealt with this some months ago, and I need to check my notes to give you an adequate answer. As for the "directly," I see your point, but I still think the back route was the most direct route. It was largely open and unobstructed. Again I'll have to check sources, but there is evidence that the O. K. Corral used the vacant area. It may not have been used to hold stock, but it could have been used to store hay or park wagons or hold other equipment used by the corral.
I'll get back to you on this one, but I have an appointment that I have to keep. It'll have to be later.
Best!
Gary