Both should be accepted as the English "you" can refer to one person or a group of people.
Thanks. So in all cases where it is "your" we are translating, it can be either?
Yes, or at least it should be. If you'll ever encounter a sentence where it won't work both ways, you can report it.
Although, to be clear, there may be sentences which will make it clear if it's singular or plural, for example "You all have coffee" - here, because of the "all" part it's obvious that it's about a group of people so it could only be "Wszyscy macie kawę".
What exactly is the difference between twoj and wasz? Are they interchangeable or is there some subtle thing like whether or not the noun is personal?
There's a huge difference, actually. "Twój" is something you'll say to one single person while "wasz" to a group of people.
Now, theoretically, the word "Wasz" could also be used as a polite form when talking to one person, but practically it's not really used like this anymore. You would have to speak to some royal family member to address one person with "Wasz" and you'd only see this in old literature, usually you'll just use "Pan" or "Pani" (Sir/Madam) as a polite form.