The cons of a backpack is that you really can't do many windows with one, atleast, not with a proper 2lpm flowrate.
Also, most of us do not look forward to lugging 25l canisters around to fill it up with.
If you are half serious about wfp window cleaning, a van mount is the only way forward. Higher initial investment, but very good returns. You are trying to make work easier by going wfp. For me a setup like this is out of the question.
Ps, the pole is not fantastic, and the brush is poor, it's heavy and doesn't feel right on a window. Also, the 24ft model isn't the smallest pole when closed, so when doing ground floor windows, it might be awkward.
DI container is only helpful when your in a soft (read very soft) water area. Anything up from 40ppm I would go RO.
You don't need a neck protector quite frankly.