UseDots doesn't change your spell lineup so if you don't have any memorized either in the default or you are in byos, its not going to cast them.
the stance just changes your spell loadout - it doesn't change any fucntionality or priority
you should memorize plague at lvl 54
this will cast it on your ma's target if you are above your shmdotmanamin and if the mob still has a good chunk of health left (unless named, then it will still cast a much lower health).
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I did notice an issue with it not memorizing cripple at lvl 54, however. so I'll update that.