You can find a thorough defence of retaining the remnant of the old library building at Willesden Green here. I am not sure that the legal reasoning towards the end actually holds up, but on the whole it is as carefully argued a case as anyone could present.
The author has previously contacted me about the interim arrangements. I recently went up to see these for myself. They are in two parts. There is quite a sizable space that the Council leases on Grange Road. This contains the current archive in a basement area, and would house an appointment archive service during the construction period. It would also have room for a number of computer terminals and study spaces. The second part of the interim arrangements would be a similarly sized building in the Willesden area. I can't reveal the location until any deal is finalised, but I liked the look of it. Together they won't provide an exact equivalent to the current library, but they are much more extensive than some people seem to assume.