Star Tours transfer seat doesn‘t have anything to open; it is the seat closest to the entrance door on the front row with an aisle next to it. The armrest on the aisle side can be folded up, which can make it easier to board. If you want to use that seat and the armrest feature, tell the CM.
This website has details.
You wont be able to use a stool for boarding attractions.
If she needs help getting over the side, you can ask to use the Transfer Assist Device (also called Transfer Tier). They are available on request for many of the hard-to-get-over-side attractions. It looks like a small set of steps or bench that hooks over the side of the ride vehicle. It’s not a set up steps to walk up - it’s meant for wheelchair users who can’t stand/can’t stand securely to sit on.
The first step is about wheelchair seat height and the next steps go over the side of the ride car, then down to the seat. Wheelchair users park their wheelchair next to the first step, then transfer over to it and ‘bump’ up and down the steps into the ride vehicle. Getting out is done the opposite way.
Someone not using a wheelchair can do the same thing - they would just sit on the first step instead of transferring from a wheelchair.