Paint out of the bottle is usually too thick for applying on to parts. So thinning is necessary. The thinning ratios are different for hand brushing and for airbrushing. Since the paint needs to be fairly thin to spray through the airbrush decently (or it will clog). I do not hand paint so I cannot suggest the proper thinning ratio. For airbrushing I typically thin at a ratio of 3 parts thinner to 2 parts paint; but this changes due to inconsistent thicknesses of the paint. The resultant mixture is usually the consistence of milk (again, for airbrushing)
Yes, paint thinner can be used to clean up panel lines, but you need to read up on that particular technique, and I suggest you read that here
There's information about the steps needed. If not followed directly, you risk running into a big mess.
For seam lines, read this: Applying glue for seam lines
Most of these glues designed for fixing seamlines have a brush included. I have switched to using tamiya extra thin cement, which also has a brush for application.
And finally, there's a stickied thread: Model techniques
and there you can find a torrent link for my power point based tutorial... granted it's only pictures, but some of the videos should help.