Can you share which version of 6 you're migrating to? The stack trace in the original post looks like a bug. For good measure can we take a look at your kibana config? Double checking indentation may help.
Other notes - tribe should work in 6 but is deprecated in favor of cross cluster cluster search, if that's an option.
Index creation issues can be caused by creating the index on the tribe node instead of directly to one of the clusters. Index migrations are probably getting in the way, so we may have to run migrations directly on one of the clusters first and then configure tribe.