Still a crappy season overall. I just hope they don't waste Deb witnessing it like they wasted Rita dying next season.
I thought the aftermath of Rita's death was handled about as believably as possible (Unless you're referring to how Dexter's relationship to Trinity was kind of brushed under the carpet by the cops, in which case I'd agree).
As for Debra, there are all kinds of ways they could have her deal with it. She could go into denial, pretending everything is normal for as long as possible. Or she could go catatonic and never leave her house...
Unfortunately it sounds like they want to pursue this relationship idea, so she'll probably just decide "fuck it, I'm in love with him, and he's all I have, so I'm just gonna go along with it."