I've always found scissors are better for trimming overlap like that, as for the gaps, facing them to the track is a good option, otherwise I'd go with what Dave suggested and just wrap another layer of thin paper around the outside. You can either paint it or scan and print something of the same colour.
As for mounting the wheels, I've never built a tank so I'm not sure how the wheel and bogie sections go together but I've built more than a few multi axle vehicles. What I like to do is build a base for the model to sit on so that it's at the height it will sit with the wheels attached, then mount the wheels from the sides, I've attached a pic of how I've tackled this on my current build... I just built two little stands to hold it at the right height. Then it's just a matter of gluing the wheel to the axle and pushing something against it to hold it up right while the glue dries.
