Somehow in the fast version'I hear 'du Var min bästa vän'. Am I the only one who hears the 'v' sound? Or is something actually there?
The u-vowel can sometimes have a small off-glide that sounds something like a bilabial approximant [β] as in Spanish ’curva’ or something similar to English [w]. That might be what you’re hearing.
Yes, objects "owned" by another through a possessive are treated as definites.
en frände is an archaic word meaning 'relative' or 'kin'. It's used today in compound nouns such as meningsfrände 'someone who shares someone's opinions'
What would the translation be if I am referring to more than one friend as best. Like, you (plural) are my best friends. Bästa sounds like an adjective for plural subject
You are right. I am not sure why it is bästa here. It may be because min makes it definite. Perhaps someone can shed some light on that. As for your other question that would be Ni är mina bästa vänner.