I hope you have success and are able to have your whole party go. I'd work up some sort of Plan B (just in case) which it sounds like you are already doing. Maybe if things keep going well COVID-19 wise (lower numbers, fewer hospitalizations, more vaccinations), capacity will increase at the parks making getting that extra ticket easier.
Not for the parks, but one time I had reservations for a party of 10 to go to a sold out Hoop de Doo review, and then a cousin I wasn't expecting joined us for the vacation. I decided to have all eleven of us go over and that I would be the one to skip and meet others after if only 10 could go. To our surprise, another party of 11 who were to be seated right in the same section we were in had one person who couldn't go and only had 10, so Disney just swapped the tables for our two parties. We bought the extra ticket and the other family even got their money back (win, win, for all). I will say, though, for sold out Christmas parties I have never been able to get an extra person in.
Your post and other similar posts helped me make the decision for our trip in March 2022 to reserve in advance for everyone who might go. So I do want to thank you for sharing your experience as it does help others.
Hearing about all the issues people were having getting reservations, I went ahead and bought tickets and made reservations for everyone in our party who plans to go (even though there is the possibility that two might not be able to -- school schedule for one unknown / 85 year old parent will probably feel up to the trip, but could change mind). I figure it will be easier to cancel last minute than add last minute. (A reason that I can do this is that you can change the name on an unused ticket, and with family in the area and the fact that we go down every year or every other year, there is no question that tickets will be able to be used at some point.). Also, for Disney, these are just two day tickets -- much less of a commitment than someone doing a longer stay. All someone would need to do is to twist my little pinky and I'd be up to do a mini trip to use extra two day Disney tickets and do a mini get-away weekend or something like that and get to visit family too.
I hope you and your family have a great trip no matter what.