If you live in Canada, you will be severely handicapped by the much worse signup bonuses for cards in Canada.

I am very much not an expert on redemptions, but I'm not sure that UA offers a stopver in each direction plus an open jaw. AC might but as you observed, fees on AC are much higher.

Yes, all the points can be in your account, and in fact should be, since your family members won't come close to having enough points to do anything useful.

It would help to know what your typical CC monthly spend is.

Air Canada often runs transfer promotions. They are not incredibly rich, but for example, you can exchange 100K Amex points for 120-125K AP points. They usually come up at the end of the year. They also have periodic other types of promotions. They had one a few months ago where you could get 16,500 bonus miles basically by going to Sobey's and Esso. (Lots of people were going to Esso every day and buying $3 worth of gas at a time.) Right now they have a hotel transfer promo. So you might also consider getting one or more hotel cards that allow transfers to AP. If perchance you had a Marriott card right now with the 70K signup bonus, you could have converted that to AP for 25K + 5K (I think) bonus.

If you can get USA cards, it's worthwhile to get the UA explorer and Chase Sapphire Preferred; Chase points can be converted to United. You could easily run up 100K points on UA and get one ticket from that. Note that there are tricks to getting the best signup bonus on the Explorer card. (Have at least 1UA point in your account and be logged into your United account, then look for the link.)

Finally, don't assume that just because you have 400K points on *A that you will be able to find four award seats on the same flight, even if you do so 11 months in advance.