I agree: Jonathan Groff as Marvin would be perfection.
Is Annaleigh really too young for Trina? Weren't she and Marvin high school sweethearts? Meaning they could both be right around 30 (since Jason is being bar-mitzvahed). I think she'd be wonderful in the role.
As for the rest, I can't really decide. I agree Aaron Lazar would be great as Mendel simply because I could buy him as half-Jewish and he really does look better than a donut. I actually think Christian Borle would be a better Mendel than Marvin (older and more serious than Groff).
The lesbians...one butch, one girly; maybe Lea Delaria makes her return to Broadway as Dr. Charlotte and maybe Elizabeth Stanley as Cordelia.
"Contentment, it seems, simply happens. It appears accompanied by no bravos and no tears."