Hi guys, this is quite the interesting topic.
Well, it's definitely true that the two char species share many habitats and are sometimes found to hybridize. Along the coast, small river systems and even large rivers systems which may be far away but still connected to the sea, may often have dollies. As clarki has quoted, dollies are anadromous and freshwater species.
In more interior waterways is where bull trout are often found as they are predominately a *freshwater species*. However, in BC there are many areas of overlap and due to the glacial history, and head water transfers from the Columbia River, and the Fraser River, and the Fraser River with adjacent tributaries from the coast and interior regions we see these two species coexist. More information on BC's glacial history and the native fish species and distribution can be found in the below book and subsequent chapters:
McPhail, J.D., and Lindsey, C.C., (1986). Zoogeography of the freshwater fishes of Cascadia (the Columbia system and rivers north of the Stikine). The zoogeography of North American freshwater fishes. C. H. Hocutt, and Wiley, E.O. John Wiley & Sons, New York: 615-637.
I wrote bull trout are predominately a *freshwater species* because they have in fact been found to migrate into the sea. There's been some telemetry work done and some genetic work which suggests so. Also, such life history information have been updated in McPhail, J.D, (2007). The Freshwater Fishes of British Columbia. The University of Alberta Press.
As Rodney had quoted from the hybridization study, there lies an example of the two species living together and coping to survive. There are a few published papers documenting the hybridization and co-existence of the two species in nature. In fact, the paper found here:
http://www.zoology.ubc.ca/~etaylor/taylorandcostello.pdfcan be found quite interesting as it talks about bull trout movement from tributaries to major river systems and back, as well as hybrids found in BC water systems.
As for Rodney's questions, yes those are interesting questions and I don't know the answers off the top of my head, but many people have their own theories and ideas of studies to try to answer such questions. It's just that it also needs some funding to do so ;-)
I hope that helps but it seems that it's getting late and I'm just rambling on.....