You wear business attire throughout EPs and only start wearing the uniform when you commence service training. You won't be sharing an apartment with anyone else. MAM will provide all the flight and accommodation details a little closer to the date; don't stress :-)
As long as you can swim, you will pass the swimming component. It just involves swimming from one end of the pool to the other, then dragging someone back with you. It doesn't matter how long you take, how good you are at swimming or what kind of stroke you use.
Dental X-Rays are used for identification purposes in the case of something unthinkable happening.