Tom
We had to prop the door open for a while, as we also had the problem that cat #1 wouldn't let cat #2 in!
This is a very good point especially given that it's a front leg that's missing.
Our cat managed fine using the cat flap but he was missing a back leg which generally speaking is easier to manage.
Yes, that is a consideration, especially at 1000CHF a pop
but surely he will push with his head?
Here is an experiment for you. Go to the nearest door, have your hubby apply apply enough force to the door that it takes some effort to keep it open. Now stand on one leg, push the door open with your head, and hop 20cm up over the doorsill to get into the room.
Now imagine that the hinges were at the top of the doorframe and the weight of the door was pushing down on your head while you were hopping over the sill.
Yes he will push the flap open with his head as all cats do but it will be the logistics of getting his front half through whilst balancing his body weight that will be tricky.
They usually put their head through and then the front paws one at a time whilst holding their front end up with the other one. With only one front leg once he lifts it up to put it through the flap he won't have anything holding his front end up and will have to find a way to hold himself up somehow probably by resting his head somewhere. It can be done but it's a tricky balancing act that he'll need practice to master.
And please ensure you video the above experiment and post here