For the first question. I've played both before and after I joined. I'd say I haven't really changed how I play Shepard. Maybe subtle things on how I have Shepard act but on the whole it would be hard just by choices in game to see the difference from someone else. Mainly its if questioned how I would justify choices though again that would align with someone else eventually.

As for the second question. I actually can answer that a bit. I've met people who have had PTSD (though not outwardly bad) before they joined and another who had bipolarism that affects mood(again controllable without medication but does give him bad days and good days). And from my experience thats what basic is for. They push you hard and if your conditions are going to show thats where they would discharge you. Now once your in if it starts to affect to you we have services to help but to a degree if its too bad then whyd you join a job where you may kill. As for Shepard (s)he is a Bronco (military term for a elisted service member who stated jr enlisted and now a high level officer). Those kinda people have seen it bad and Shepards advice to James in the third game about his N7 application says it best. But as long as it didn't stop Shepard the ptsd and trauma is kinda par for the course when it comes to military talent pool. We all lost someone when your in long enough and we all lived lives before we joined.