1. A five stop subway is not going to have much of an impact on 401 traffic because it is completely useless for suburb to suburb drivers right now as almost everyone has to take several buses at either end to use the Sheppard subway right now. I notice that many people using the Sheppard subway are going downtown even though the vast majority of 401 drivers using the section of the 401 running parallel are not going downtown. I would expect a Downsview-Scarborough Centre subway to serve far more suburb to suburb traffic and far more North York Centre to suburb traffic. I would expect to see a slight drop (not enormous) drop in 401 traffic when such an extension opens, and a big increase in subway traffic. The big difference would most likely be that the subway would prevent the traffic from getting even worse than it already is.
2. I think that the main reason people would switch to using the subway is to avoid having to drive in severe traffic congestion. People who are conveniently served by the subway (e.g. work at the Airport Corporate Centre office park and live at Eglinton and Bayview) would use the subway to avoid heavy traffic. People who are less conveniently served by the subway (e.g. Islington and Dixon to Hurontario and 401) would be more likely to continue to drive in heavy traffic. Parking fees would probably only play a minor role (mostly for people working in North York Centre, Yonge/York Mills & Yonge/Eginton, if the city cuts taxes and employment growth occurs there).
3. My assumption is that buses (e.g. #7 & 35) would link a station near Eglinton/Renforth with the various office park buildings, other office parks in Mississauga such as the ones near 401/Hurontario and Meadowvale, and Square One.