Great links Doublej!!
I worked around nursing homes for 25 years and all the disadvantages of the walk-in tub listed are very true. There is no cheap, easy, small solution to accessible bathing at home.
Your best option for safely bathing at home may be the use of a ceiling mounted electric lift on a short track from outside to inside the tub like some hospitals and nursing homes use (check the structural capacity of the ceiling first). She should be able to be to use it without assistance or wouldn't take up much space, but being naked older users can find it can be a scary experience (like a carnival ride). Just hope the lights don't go off (you do have an automatic generator, right?) or the lift fails if bathing alone.
The roll in shower is a very good long term solution if you can give up on the idea of bathing. One of my wheelchair bound respite clients has a large bathroom with handicap sink, toilet, and roll in shower that is open on two sides with grab bars for assisted showering - a very good solution for his needs. Of course there are also low curbed pre-fabricated shower stalls with built-in handrails available and if needed a shower bench can be added. That's what my folks did.