The maximum output of the FE18 at -10C is about 22,000BTU/hr, and drops to about 15,000BTU/hr @ -25C, so it's not that different from the output of your Oil Miser. The FE12 delivers about 13,600BTU/hr @ -10C running full-out.
According to weatherspark.com datasets -10C is about the midwinter mean daily LOW temp, not the binned hourly mean for Port Hood. The binned hourly mean is closer to -5C, but your 99% outside design temp is probably about -19C (slightly warmer than New Glasgow) At -20C the output of the FE18 is about 18KBTU/hr, and the FE12 is good for maybe 10K.
All but the smallest wood stoves can average outputs over 15KBTU/hr in an overnight burn, so the woodstove may have been important for handling your peak loads on the coldest days. If the plan is to always use the wood stove during colder weather the FE12 would probably handle your average load (at -5C) just fine on it's own. But if you hope to stay warm with just the mini-split during a -20C night, the FE18 would be a better bet, and it just might cover it.
To get a handle on the heat load calculations, if it's a plaster wall with plank sheathing and wood clapboard or shingles, with 6-7" of depth in the wall losses at -20C in a +20C room (a 40C delta-T) are about 3.5 BTU/hr per square foot of exterior wall area, maybe a little less if the beams are widely spaced.
Clear double pane windows (no low-E) lose between 35-40 BTU/hr per square foot of total framed window area (sash and framing included) A typical double-hung window is about 10-11 square feet, but measure them. (Yes, an order of magnitude lossier than the walls, per square foot.)
With 6" of blown cellulose or rock wool you're again looking at about 3.5 BTU/hr per square foot of attic floor. If the upstairs is doored off and basically unheated, dropping to near 0 when it's -20C out, it's probably more like 2 BTU/foot for the combined upstairs wall area + attic floor area- it depends on just how cold it really runs up there at the temperature extremes.
For the floor losses add up the total square feet of the above grade portion of the foundation, and figure on maybe 50 BTU per square foot. It depends a lot on just how cold the basement gets, but it's likely to be a significant fraction of the total heat load.
This is an extremely crude estimate- the true loads could be either higher or lower, but it won't be 2x higher, nor will it be half. I'm sure the wood stove could handle the entire load by itself if you were keeping it stoked overnight, unless it's the worlds smallest parlor stove.