The mystery is an action
and as a result, it triggers his talent attack before the mystery, however that attack is not an "action" in and of itself

Therefore, he is behaving as he should.

Which is kinda silly, that this can be triggered twice... the only time that comes into play is if he is granted an extra attack by Bee or Earth main or something. Maybe in the original design the mystery was prompt? That would make more sense....

You could Jirobo, Jugo, Earth main, Ukon&Sakon.
The first 3 are all earth attribute, so between the cursed mark buff and Jirobo earth barrier, they'd get quite a big bonus.
And then earth main uses his 2x normal attack mystery, so they all hit hard and frequent.
Or alternatively, Yamato instead of Jirobo.

I mean, you have to use some main, so could very well pick one that works with Ukon&Sakon.

I do think that Earth main jutsu cost chakra thou, so does barrier, so unfortunate you can't use both on round 1.... not sure how that would work out really...