To my most faithful student, Twilight Sparkle,
I find it commendable you have taken interest in researching topics which have proven important to you and your friends in the past. No, Pinkie Pie is still not up for a retest on her demon summoning license. Regarding Cadence...
Twilight, Princess Luna and myself have dominion over the Night and Day respectively. In a plane absent either, we have very little outright power that does not tie instead into political power, and a very long time to amass magical knowledge.
Cadence on the other hoof has dominion over Love (and, as much as she downplays it, many of the aspects thereof). Cadence is perfectly capable of dealing with any being impersonating her that she does not want to impersonate her on her own, and I highly suggest not testing that unless you intend to do so from a great distance.
Incidentally you and Anon are not alone in underestimating Princess Cadence, and also bear in mind that the Crystal Empire may have different rules and regulations regarding interplanar law.
Your loving mentor,
Princess Celestia