His in-house back-end devs worry about feeding the proper content into my themes. I don’t really know anything about their back-end and it’s not really my job to know.
Worry about feeding the proper content? Well let them provide you with samples of that content so you can be informed. I would imagine they need to give you something, you can’t be designing in the dark.
The way it’s been described to me is a cobbled-together system primarily written in PHP
It’s not part of your scope so ignore it. You’re only designing the feel. Let them worry about how to bring the content into proper places from their Db.
My client wants every theme to have that uber custom look… but he wants a lot of them developed relatively fast.
I also want a place in Dubai with nice ocean view, by next week. What you’ll think that will cost?
I’m kind of thinking Bootstrap on this one
Take a step back and don’t start thinking about the technical issues, just yet. Get as much info as possible about the problem they’re trying to solve. Then do some research, there will most likely be design review of some sort, so don’t put the cart before the horse.