I have a similar problem...my toes stick out too far to the sides, & they never used to do that..I sort of think my trainer is wanting them facing straight forward, as your old trainer conditioned you to do...however, I agree with your new trainer that it DOES seem to create a somewhat unnatural position to point the toes completely parallel to the horses side...
Because my NATURAL leg/hip position is the opposite of bowlegged, I feel all that forcing of my toes perfectly straight, especially around a large barreled horse, and only being 5'1" myself, causes a lot of trouble naturally gripping with my upper inner thighs. Something for me to think about, and talk to my trainer about. So glad you brought it up! :0)
I tend to think YOUR problem WILL BE OVERCOME with increased time in the saddle and strengthening the healed portion of your ankle post-operatively. That can take some time since all of the connections in the ankle that were cut or torn and repaired are new connections, and thus you are creating a whole new set of muscles to work in tandem, just as a never ridden on ankle would be experiencing...I realize I did a crappy job of explaining that, been up all night at work..apologies!
My point is that you should try not to get frustrated or to push that ankle too hard..if it is getting wobbly after 15", that is your body telling you that your ankle has done all it can for that session. Perhaps at that point, you could do no-stirrup work out something else to relax the junction of the ankle? I imagine, with continued riding and if you employ the exercises mentioned by Tiny and others, you will find that each time you ride, you can go a bit longer without wobbling in that ankle! I'm sure your trainer knows you aren't being disagreeable but just need more time to build that ankle slowly! :0)
Best of luck to you!
"The argument from intimidation shows nothing but intellectual impotence." Ayn Rand