Many non-towered airports have diagrams in the Chart Supplement, and there is an airport diagram for X26 but not for X59. The difference appears to be that X26 has instrument approaches but X59 doesn't. It looks to me like the airport diagrams are considered part of the terminal procedures.
But I have to say that the FAA is as clear as mud on this. The airport diagrams legend says:
selected towered airport diagrams have been published in the Airport
Diagram section of the A/FD
But obviously that's completely wrong, because a) many non-towered airport diagrams are in there, and b) the A/FD doesn't exist any more.
By inspection, it looks to me like airports without an instrument approach have no airport diagram (X59, X25, 2RR) and ones with an approach do have one (X26, X07, AVO). But I only spent a few minutes looking, so I may well be completely wrong on this.
As for your question about the order of diagrams, the legend says:
Diagrams will be listed alphabetically by associated city and airport