Zeke,
I have installed a few Timneys in my Mod. 70 Winchesters, and had to do the same thing to the sear openings in the receivers. It's nice to be able to clamp the action securely, but it is not a necessity.
Use small needle files: the square profile works for me, and be very patient as you work. It's not brain surgery, you can do it! :-) Check the clearance of the sear to insure that there is no binding. I'm sure that you are going to like the Timney!!
As for you question on the two stage trigger, I haven't done it, so I can't comment on that. Good shootin' to you sir!