I think ebikes are modeled after mopeds. Gas Tanks and Batteries just so happen to be in the same place as batteries are on ebikes now. On my rack mounted ebike, the battery disconnects some times because the battery moves due to bumps in the road. The fastner wears out. I would go with a downtube or inclined tube mount. Most ebike websites have buyers guides that are very informative.
An ebike for your wife. If you're open you could get a class 3 ebike for your wife that will go up to 28 MPH with pedal assist. My graphic for ebikes is PeeWee Herman's big adventure where Pee Wee is passing the racers. It will make a big difference for your wife and make it fun for her. You're an average ebike customer if that makes a difference.
I have enclosed some video buyer guides. Good luck. The best advice that I can give to you is go and test ride and ebike. Test riding will answer a lot of your questions.
https://www.youtube.com/watch?v=8BghrrhmQvU
https://youtu.be/qfViBJa6vx8