I don't think the PFI is at YEG is in any danger at this moment. The airlines like preclearance - especially the US carriers - because it allows much quicker, easier, all around better connections at the US hubs - plus it allows airlines to offer same-plane service to other destinations. There's still enough activity on the MSP, SEA and DEN runs to preclearance going, at least for the time being.
The sad part about all this is that I don't think it really matters how the YEG originating routes do. With Alaska's new YYC-LAX route, I'd suggest that AC probably took a bit of a hit on YYC-LAX. It couldn't exactly cut back on YYC-LAX, so it would cut YEG-LAX to fill the passenger deficit out of YYC. Can't say it definitively, but seems to me likely given that at least anecdotally the loads on YEG-LAX were higher than YYC-LAX (although yields probably not as good).
Oh, and finally on the possibility of AS flying to YEG. I don't see why not, but wouldn't hold my breath - the issue of airline service into Edmonton has been an issue as long as I can remember, and that doesn't appear to be about to change. And the mayors and airport officials have always had the same hot air filled commentary that requires some skepticism.