If it were me I would take Grandma up on the offer...especially if it means your DS will be in his own home while youre at work. That way your not having to continue at the FDC while finding another place for him. And being at home with Grandma he isnt having to re-adjust to new surrounding just to have it change again.
Like the other ladies have said go with your gut....and what I have learnt very quickly is when it comes to your baby there is no such thing as too many questions!!!
Goodluck