Does marinating hinder brining?
First: the information that "marinades only penetrate 1/8 inch" appears to be entirely from the show America's Test Kitchen (ATK); I reviewed multiple articles on it on the internet, and all of them cited ATK or didn't cite any source.  I mention this because ATK has been wrong before due to flawed test conditions, so maybe some judicious testing of your own is in order.  Assuming that ATK is correct, though, here's answers to your questions:


With marination, only the salt will affect the meat more than 1/8 inch deep.  However, the other flavors will affect that top 1/8 inch, so if you're eating that portion you still care.  Also, some marinating techniques like lechon asado involve piercing the meat many times, which lets the marinade penetrate more of the meat by increasing the surface area. 
For a dry brine, having ingredients other than salt would only interfere with the brining process if the other ingredients were on thickly enough to keep the salt from contacting the surface.  Since dry brining rarely involves a 100% salt coverage (although there are a few that do), this seems unlikely to be a real problem.
Sugar will actually have a similar effect on meat as salt does; both denature proteins and increase moisture retention.  Of course, sugar is rarely used to brine meat, but it is used for fish.  Serious tenderizers, like bromelain from pineapple, may also penetrate the meat -- certainly we believed it did for South Texas fajita recipes -- but I haven't ever seen an actual scientific test of this.