I'm kind of glad, in a way, that they never found out. It could have gone one of two ways, they thought "wow it was Jessie and she was actually good at it, good for her!" or "It was JESSIE? She must have cheated to get as far as she did". And because Jessie is part of Team Rocket, I think it would have been the latter. I love Jessie so I wouldn't have enjoyed seeing Ash&Co. find out it was her only to be skeptical of how she got so far. I think they would probably throw out any admiration they had for Jessilina if they found out it was actually Jessie.
I don't know about the contest thing. Do we even know how contests are going to work in Isshu yet? If they do it differently then maybe she (or maybe James this time?) will have something to do with them again.