I think that would work regarding the free supercharging. The complication I see is that you both want to be seeing your Powerwall information and your car information, and it is a bit inconvenient to be logging in and out of Tesla accounts. We inadvertently ended up with our solar/Powerwalls on one account and our cars on a different Tesla account. So we have granted authorized access for the cars to the Solar/Powerwall account, and that is the one we actively use each day for the cars too. The authorized account can do everything that the owner account can do except purchase upgrades for the car. BTW my name is on both Tesla accounts… nothing in their process seems to exclude multiple accounts. All that is needed is multiple emails.
So for your specific situation, I would recommend to keep your daughter’s model X on the current account, but that you each start new accounts for your Powerwalls, and then each of your new accounts could be authorized to use/see the existing model X. I suppose she could just keep using the one single account for everything, and my only real advice is for you to proceed with your plan to open a new account… and planning to operate the car from it as an authorized user.