I’d give serious thought to starting at Les Houches, it’s logistically MUCH easier. That way you can get one of the millions of minibus shuttles (e.g. Easybus, Cham express, Chamvan, Mountain drop-offs etc.) from GVA to Chamonix/Les Houches which only takes an hour, vs. faffing around getting train to Martigny and bus up to Orsieres/Champex. This also enables you to have a more flexible schedule because if things go slower, as long as you get to the end of my day 6 by your day 8, you can be back in Geneva that evening. If things go really pear-shaped its easy to bail at Courmayeur and get one of the 4 daily buses back to Chamonix.
My schedule from a few years ago is below; we camped everywhere and carried all our gear.
1 Les Houches – Refuge La Balme (camping nearby, food shops in Les Contamines on the way, so no need to carry food up/down the hill at Les Houches). You could conceivably to this on your arrival day.
2 La Balme to Elisabetta (camping on the wide plain just before Elisabetta)
3 Elisabetta to Ref. Bertone (camping allowed just above and behind the refuge, you can get nice meals at the refuge)
4 Bertone to La Fouly (nice camping at La Fouly, supermarket, restaurants etc.)
5 La Fouly to Relais d’arpette (30 mins after Champex) – nice hotel with dorms and large camping area, restaurant
6 Relais d’arpette to Tre-le-champ (between le tour and Argentiere) – nice camping with attached hotel (restaurant), supermarket in Argentiere (20-30mins walk)
7 Tre-le-champ to Lac Blanc or anywhere in the mountains above Flegere (allowed to camp from dusk to dawn in mountains above Chamonix) (this was only a half day, we spent the morning shopping and eating in Argentiere)
8 Lac Blanc/Flegere to Les Houches
If you were slower than expected, then once you’ve got to Tre-Le-Champ its easy to get a bus or train (or walk 2 hours along a flat trail on the valley bottom) to Chamonix/Les Houches, and get the minibus back to Geneva.
More recently, I’ve run it in 3 days on the following schedule:
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.