OP: I think your original notion with the ladies, small carbon frame Felt is spot on.
Looking to solve a different problem a few years ago, I bought a Felt F95 Jr. for my daughter. She has Down syndrome, is 4'9" and will be there forever. The advantages I saw at the time were the 650c wheels, the narrower bars and the smaller shift and brake levers. I also had interrupter brakes put on.
At the end of the day, the bike was a bit twitchy, surprisingly heavy, and with less than stellar components. No knock on Felt-the bike was exactly what it was supoposed to be, but I switched her over to a Trek Domane WSD last year, and did a flat bar conversion, which is working out great.
The best news, and why I agree with your plan, is we have used the Felt as a loaner for house guests, and adjusted it to fit riders as big as 5'6". Perhaps not optimal, but viable for rides up to 30 miles or more.
So, I could see at least 4 years of happy riding for your daughter, on a light bike with great components, followed by four more years for daughter 2. That sounds like a wise investment to me, not a waste at all.