I don't think trimming the posts would cause your problem. I think gently pulling up on the ear while twisting the post as you are pushing them down in the ear will help getting the post down where you need them.
As far as backer rod size, maybe try 3/8"
In the beginning posting is new for both of you and like they say practice makes perfect. It won't be long before both of you get used to it and are sailing right through it.
Deep breath, relax and keep it fun, you'll be an expert before you know it!