only problem with cleaning upstairs wfp and downstairs trad. would be that it looks like you yourself dont trust the wfp system. if you did you'd have no problem doing the whole house wfp and cos they feel you dont trust the system they wont, and will look out for errors.
i think you'd be best turning up on the morning of starting and just work as you would normally and when you speak to them, epolagise on being late and explain some of the reasons. if they ask about the system, explain as you work tell them it may take a few cleans to get perfect results, but when you do they will be perfect and will stay cleaner for the whole month instead of just a week or two.
i work trad, so i havent had to go through explaining to people, but thats just the way i'd do things if i was in your situation.
spend time getting to know each customer a little too. just because you have a lot of work to get around, spend 5 minutes talking to a customer if they talk to you. do this while working if possible - but try not to rush off. you'll benefit from talking to customers and making friends and building trust with them the first few times, and occasionally after that just o keep a good relationship. this way they are less likely to drop you for a lower priced window cleaner because you are seen as a friend and not just the man who cleans their windows
good luck