How do I salvage a 1-week old research internship offer, if possible?
The message you sent has a sufficiently professional tone that I wouldn't worry about it. While it would be better in general to thank the person for their interest it doesn't seem like a big issue here. In particular since you were a bit self deprecating in your tone. 

But a couple of days is nothing in academic time. That is always true, but especially now when people may be struggling with finding creative ways to teach without having student present. Since the offer wouldn't become "live" until August you have some time to wait.