The Starly might be at a higher level than the Onix and it's just never explained. Or, more likely, Mitsumi didn't think the Onix would be that hard to calm down and didn't want to use one of her higher-leveled Pokémon.
Also, it was allegedly part of her training as part of Team Galactic to fight using weaker Pokemon than her opponent. (And apparently, it's worked well for her.)
So...why WAS Cyrus at that contest anyway? Did he find out that Mitsumi was going to be there and race over from wherever he had been? Did he just happen to attend and oh look, there she is?
Probably the second option. I'm not sure how applicable this is to DPA, but you do see Cyrus in Mt. Coronet in the games, which is right next to Hearthome City, so... It's not too much of a stretch to assume Cyrus could have been over in Mt. Coronet and then just decided to go over to the contest afterwards.
How was Hareta raised? It seems like Rowan just let someone barely older than a toddler loose in a forest by himself, all day every day.