Hahahaha.... Funny girls. I know what you are saying though. My mom is in her late 60s, and is "old" (not in age, but in body and mentality). But my mom-in-law is in her early 50s and has a totally different persona, so seems MUCH younger.
If they have the flexibility to take the 2 weeks off whenever you need, then I think that is the best plan. However, if that is scaring people off, then I would absolutely offer Gramma as back-up. Make sure that Gramma knows what this will entail, as well as prospective parents. Are you going to be totally off and it will be at Gramma's house? Or at your house and Gramma will just be doing the majority of the work?
And then, I agree, discuss (esp with Gramma) the options if you need a section, etc.
Good luck with filling the space!!!