PGmc Prefix

Will this then work: fra- = (far) away (from), off; fur- = fore-, before; fer- = completely, fully, up?

I don't think that is right. *fra- also meant "completely". I think it is best described as a "perfective" prefix.

Going back to PIE, *fra- comes from *pro-, *fur- from *pr- (related to *fura, *furi, German für) and *fer- from *per(i)- (Latin per).

Also, if you consider the regular operations of sound laws, Germanic unstressed -er- remains and doesn't become -ir- (while unstressed -e- otherwise does become -i-). But in most words -er- later appears as -ar-, especially in West Germanic. So it's possible for West Germanic *far- to derive from earlier *fer-. Gothic *fair- can come from either *fer- or *fir-. The appearance of *fir- in West Germanic doesn't have to be significant, because *ta also appears as *ti and *ga- as *gi-, so it seems that prefixes involving *a generally became merged with *i fairly early already. The same could have happened with *fur- too.

Also compare *twiz- and *tuz-, which may give some more insight in how vowels developed in prefixes.

Thanks :)