I have a question about how to work with multiple stores and only one gruntfile.js file.
Has anyone done something similar?
I've had success using the Magento 2 way of organizing their grunt workflow in Magento 1 and iterating through them for each grunt task.
If you look at https://github.com/magento/magento2/blob/develop/dev/tools/grunt/configs/themes.js
You'll see they list out each theme as well as the files specific to the theme, from here I just use require to include this file in each of my tasks (which I also store as individuals instead of just one giant grunt file). See (https://github.com/magento/magento2/tree/develop/dev/tools/grunt/configs) for reference.
We've done this for about 9 months now and have had great success with it, especially when leveraging fallback to other themes.