Good story! I'm a good say-lore, does that count...? (baaaad)
Yes, I was thinking perhaps because Murdoch had already married a Euphemia that she might have gone by Effy to lessen the confusion...altho there are some Effy/Effie births... ( Also some call Elizabeths Effy as though there aren't enough goofy variations on that forename already) It certainly seems odd that she'd always go by Effie but be called Harriet on her death cert. Interesting to see what comes to light...Perhaps a descendant will tell us what they have found sometime down the road.
Forgot to mention that there was an effort to populate the western regions of Canada... posters & propaganda were sent overseas with prompts to emigrate to Canada and to become a farmer which would attract farm lads.... or to build a railway if you were a labourer. There was a promise of land for those willing to work it.... and I am sure lots of great images with no mention of winter in the prairie farmslands! This, I am guessing would have been the prompt to go back to the far west where he saw a lot greener winters.

This from a Prairie gal, I write this in the middle of a cold spell, and looking out at banks of snow...but I can see the appeal to go to B.C. and never return, as many have! It certainly attracted many from Scotland.