I think one could argue the point that Doc was involved in a criminal syndicate known as the "Dodge City gang". All bunko artists were not stage robbers, but had other outlying contributions to the activities.

Doc was an associate and friend of Jordan Webb, a known member of the Dodge gang. He operated a bar with him and also received significant help in judicial proceedings from the leader of this group, Hoodo Brown.

After Doc turned over his saloon, for financial concerns he stayed in Las Vegas and continued to gamble and associate with all these members.

Numerous robberies occurred during this time, not to mention several stage holdups, one in which Doc's partner, Webb was charged with. These stage robberies in my opinion are a blueprint of some of the same robberies that occurred around Tombstone. If given the chance someday, it would be interesting to compare there m.o. I know they took place just outside of the town and in one instance 3 men participated in one of the stage robberies.
So you have Webb and Doc down and out in Las Vegas, part of a criminal syndicate, and Webb is arrested for stage robbery. I guess you could argue either way, just like Tombstone, that Doc was involved in it somehow or he kept himself clean? "Bunko Stuff" goes hand and hand in maintaining criminal activity and certainly could easily branch out to holdups. The holdups themselves would need others to plan and support the participants.

As to the Kate question which is very intriguing, a spouse apparently could not testify against their partner. Marieta Duarte,the wife of Pete Spence, had her testimony objected to based on that very legal principal.

One final note, Wyatt was hanging around Las Vegas with Doc on and off before finally heading out to Tombstone. He also kept himself above suspicion. Did Wyatt ever hold a law position in Las Vegas?