That's the sort of thing real life unfortunately does...
It's years since I've read Iliad book V or really given it attention otherwise, so I can't help much. The basic idea I think is that the Achaeans must be given some success first to contrast with their bad fortune later on. I'm not aware of a particular discussion of book V being an interpolation, but since about every single line in Homer has been suspected by somebody, I wouldn't be surprised if one existed.
Personally I don't think it's interpolated. Book X is another matter. It's the only book you could just drop out without noticing any discontinuity in the narrative. There are also linguistic problems, but I'm not a competent judge.
I haven't read Graziosi's commentary but I've heard it's good. Personally, I plan to have it around for my long-planned (and long-postponed) re-reading of the Iliad. If you want a modern commentary, what alternatives do you have for book VI? You have the Cambridge commentary for books V-VIII by Kirk but I'm not sure it's very good. Then you have Graziosi. And there's one volume in the German "Gesamtkommentar" that covers book VI, but I don't know anything about it – could be just as good or better than Graziosi.