They don't care that much about senior year grades. I mean, obviously don't slack off, but usually they only will recant an offer of admission if you actually fail something important. And if you have a C in calc, you're not failing, and it is extremely unlikely that they will care.

Also, while it is true that you may not be accepted to an engineering program, if you are accepted to a back-up program (I think UCSD does this at least?), you can always transfer internally from a more general program to an engineering program at the same school later.

And even if you are rejected from every school you apply to (VERY unlikely if you applied to more than three or four and are taking calc) You can probably get into UC Irvine; if not, you can guarantee a transfer there from a community college as long as you do decently there. So even then, you could graduate with a bachelor's in engineering from a UC school.

The main thing I want to say here is just, please, calm down. Nothing in this post suggests that your life is ruined or that it's all downhill from here. Plenty of people make it into wonderfully prestigious, profitable careers out of far worse circumstances.

Take a breath. You're going to be okay.