The reason you can't find a solid answer is that there is no solid answer. I've done similar stuff with crampons strapped onto trail shoes and with full on plastic boots (in fact I used rigid plastics for MB many years ago). Mont Blanc is very condition dependent. You may have to front-point up/down sheer ice if the bergschrunds open up.
If you're going with a guide, then do what the guide says or start off on the wrong foot (no pun intended!) and risk being sent off to rent some boots. Even if 20 people respond to this saying B2 is ok, that's not going to cut it with the guide if you show up with B2 boots and he's specified B3.
In the interests of balance, Sawyers response. Personally, I'm unconvinced that their filter magically knows how to identify whether a given coliform bacterium is harmful or not, and filter only the harmful ones, as they seem to suggest. Furthermore, they very simplistically identify all E.coli as harmful, which is nonsense and makes me wonder if they employ any scientists at all.