Interesting escape from the bedroom. So Voldemort is keeping Harry alive to perform a ceremony to take his magic. I didn't think it plausible he would have kidnapped just to impersonate him. But wouldnt' the blood magic that protects Harry also have protected his "family" (at least Petunia and Dudley). The deatheaters have never attacked Privet Drive (the dementors were sent not by Deatheaters but by Umbridge and they were a couple of streets over). I don't think polyjuice potion would enable a deater to pass the wards keeping Harry (and "family") safe. That part of your plot is improbable.
Ouch. Scary, scary. Why didn't anybody follow Harry to the
shop? This leaves a lot of questions to be answered. I am
duly impressed by Harry's quick thinking, and his planning, filling his
makeshift rucksack with stuff he might need. I still wonder how
Harry will manage to escape the estate, and will he manage to retrive
his wand? Too bad I don't have the time to read more at the