Got my admission for my main major, computer science, rejected. Since this is the case, would there be any chance CPP accept me for my alternate major(mechanical engineer) or do they just disregard my alternate major and reject me entirely?
Your alternate major is also impacted so they will not consider it. The rule of thumb for any schools which consider an alternate major is to select a non-impacted as your 2nd choice. It would be best to move on and get excited about your other choices.
A rejection is a rejection. Were you accepted elsewhere?