Viking may have a "multi-year backlog" on the Twin Otter Series 400, as you say, but only because Viking can only make them at a low rate.  I don't believe they have the capital, or access to it, to do much more than they are doing now.  But I am willing to be corrected.
As for the CL-215/415 water bomber, there were no more pending orders when BBD transferred the ownership rights.  The last examples were basically built by hand in North Bay.  Only about 225 or so total were built over 25-30 years.  The trend in aerial fire-fighting is for larger landplanes carrying fire retardant: ex-airline British Aerospace 146 jets seem popular, Conair in Kelowna have converted several BBD Dash-8 Q400s.  Also DC-10s and even a 747 "Supertanker".