everyone is replaceable. this is a business. you own them nothing. you've already done your due diligence and they knew you were going to leave. they will survive without you and your fellow co-worker will forgive you. i've been in this position before and i always stayed till the end of the project so i didn't screw over my co-workers, then i bailed. you've done more than is required of you already. a lot of people would give their two weeks and run. the project sounds like a mess. it not going to get any better with you staying around for a little bit more.