Sorry but i think pants on some days and nappies on other days WILL really confuse her.
Once she is ready it shouldnt take that long anyway.
Grandma will just have to deal with it, and if she is ready and has a few days with you first it shouldnt be too bad.
However children DO have occasional accidents, often for a few years after being potty trained, so grandma may well have to deal with it anyway.
I wouldnt be using pull-ups either. they are also essentially nappies and just confuse the child.
Material underwear is what they need.
You could perhaps buy some brightbots training pants which are basicaly a flannel material with waterproof outer, which will catch a little, but not a whole wee, and will feel wet.
If its really bothering you, we have 2 bank holiday weekends in a row coming up next month. Why not wait until then (so even more ready) and you might e able to have 11 days in a row with DD. On thise 3 middle days she MUST remain in pants though, no matter who looks after her.
If she isnt pretty reliable after 11 days, then she probably wasnt actually ready in the first place.
Loads of children have issues with poo-ing, often for many months after being reliable for wee's. Assuming she has a nap, putting a nappy on for the nap (ie just before she gets into bed, and off as soon as she gets up) allows her to poo at grannies without a huge mess