We were in a similar situation as you, but with dual citizenship (for me and my daughters) and a daughter who wanted to attend a small school (what in the US would be called a liberal arts college).
I would expect the French language requirement to be the same for Bishop’s as it is for McGill. Bishop’s has quite a good languages program so there should be no problem getting the appropriate education while there if you child wants to do so. Bishop’s is an English language university in a largely bilingual town right next to a largely French speaking city. The first two people who I met who grew up in Lennoxville (where Bishop’s is located) have both told me that they started with two languages right from birth.
We visited all of Acadia, Bishop’s, Mount Allison, and St Francis Xavier, and we liked all of them. We visited most of them at least twice. Our youngest got her bachelor’s degree at one of these, and liked it quite a bit. She got good research opportunities. Her first job back here in the USA sounded to me to be very similar to what she had already been doing as an undergraduate student. First semester freshman year she had two classes with 15 or fewer students in them (although one was a third year languages class). This for example made it pretty easy to get to know her professors, which helped with getting research opportunities. Her largest class over four years had 90 students in it. More recently she has gotten accepted to and is currently attending a very good PhD program down here in the US, and appears to be well prepared for it.
I have heard good things about Carlton but I do not know much about it. I did like Ottawa in a few visits there.
The main campus of the U. of Toronto is huge. I do not know much about the other two campuses. The U of T has been claimed to be stressful with grade deflation. I have wondered how much of this is due to it being relatively easier to get into compared to top universities in the US, but not any easier to graduate from.
We have also thought about retiring in Canada. One issue is that our retirement funds are in the US, and US taxes are complicated for someone living outside the country. Another issue is that we have connections and friends here in the US, and it is just easier to stay where we are.