If they applied EP for you and it gets rejected, they can apply work permit for you instead. Shouldn't be a big problem, but do take note that some banks might not be keen to re-apply the work permit if the reject reason is due to your own issue (e.g. lack of a diploma, assuming you haven't completed diploma as you said so).

It might be best to just inform the hiring bank of your concerns and let their HR get back to you instead. At least if it fails, they are already pre-empt for it.

My mom worked with a company for barely a month under work permit and she moved to another company for better pay/work hours. Don't see any harm/issues for her.

I am holding EP and changed two jobs in the recent three months time, never gotten any issue also. I think as long the hiring company does not see any problem in hiring you, you shouldn't have much hassles though I do think you might want to give a bit more time with the current job before running - unless the new job is paying damn well or holds great exposure that you want to build on.