Humanfly,
I've already answered your first question about the dust past the DNA filter.
To answer your second question about water ingress due to the DNA cover in stead of the snorkel, all I can say is that we've been through fairly deep water and unless you dunk your bike (fall over) you'll have to submerge your Tenere very deep to get water in the air intake, snorkel or DNA cover. If you really wanted to increase wading depth even more, it would be possible to do a home made extension on the DNA cover, but by that time your battery and ECU will be under water too !
Like was said before, the snorkel only serves to quieten intake noise and will not lessen water intake into the air filter.

__________________
2014 XT660Z - Barrett pipe, DNA stage1.5, Barkbuster hand guards and handlebars, Pivot pegs, Teknik suspension, Guard-it engine protector combo, B&B rear rack, Trax topbox.