My first thought is that the thermometer is off a bit or wasn't in the fattest part of the meat - what you describe fits "the stall" perfectly but usually that occurs around 160 (or a bit lower). If you aren't familiar with it do a search and read a bit - basically it's a point where the meat begins to "sweat" and cool itself down (just like humans do). That point/temperature can last for hours. In order to speed things up most of us would recommend wrapping tightly in foil to get past that.
As for the toughness it's possible not getting to temp was the culprit - we are talking about a piece of meat that needs "low and slow" to break down and become tender. When I first started this all seemed so counterintuitive to me and when a piece of meat (shoulder, brisket, even ribs) came out tough I always wanted to blame cooking too long - assuming you are staying at low temps that is rarely the case.
Not sure how experienced you are with smoking meats but you picked in my mind the toughest piece of meat to attempt. Even after I had shoulder and ribs down perfect the first brisket I made was a disaster - I swore it off for life and it was another 3 years before I would attempt my second one.
Don't give up - once you make a great brisket there is nothing else like it. I usually don't inject or marinade (just a dry rub) but many others do. Also try wrapping in foil next time when the internal temp gets to about 160 to get past the stall. From there a few options once it get to or above 180 but allowing the meat to rest for an hour or few can only benefit you as well. Lots of good info on that here as well as other sites.
Hope this helps a bit!!
Occupation?? Part time Butcher, Chef, Microbiologist, Scientist and Meteorologist – does what pays the bills really matter?