Well, here goes with answers to some of your questions. Others who have more detailed knowledge will be able to correct me / fill in the gaps.
I don't think this is going to be a regular for the Northlander - it's a test operation from a tourism-promoting organization called Transportation Options, thanks to some special funding. See http://www.nugget.ca/ArticleDisplay.aspx?e=1690933
for some news coverage in the North Bay Nugget. The Northlander doesn't have a baggage car normally.
The "heritage" cars that ONR bought a few years ago operate regularly on the Polar Bear Express (Cochrane - Moosonee) and on the Dreamcatcher Express (North Bay - Temagami, limited runs on certain weekends in September). I haven't seen any of these domes/diners on the Northlander. A few of these cars are still waiting to be refurbished.
The "relationship" between ONR and VIA that you mention more or less ceased in 1990 or 1991(?) when VIA cancelled the overnight "Northland" between Toronto and Kapuskasing. The "closeness" of the relationship is best seen by the naming of trains - why on earth would two separate trains have almost identical names: the day train called "The Northlander" and the night train called "The Northland". Anyway, the Northland was a VIA train (operated by ONR north of North Bay) with one or two VIA sleeper cars in the consist. At the same time, VIA also operated an overnight sleeper train between Cochrane and Quebec/Montreal (the route varied over the years). I did at one time do the round trip Toronto - North Bay - Cochrane - Senneterre - Quebec City - Montreal - Toronto.
Yes, the Northlander cars are former GO Transit cars, but you'd never recognize their interiors. The seating is akin to Amtrak's Superliner seating, with almost the same seat pitch. Yes, the dining is limited, but perfectly adequate for a full day's travel. If you want to see the interior, have a look at http://www.youtube.com/watch?v=w5zfhlhhPQI
(courtesy of northlanderdude).
I hope this helps. Anyone on this forum is welcome to correct any inadvertent errors here.