So many good questions and issues. Let me take a stab at some of them:
Agents: All aspiring authors should check out writerbeware.com for good tips on how to avoid phony agents and publishers. Sadly, you don't need a license or any sort of qualifications to call yourself an agent. And all agents are not the same. Ideally, you want somebody with experience, connections, and a good reputation.
True story: there was an agent years ago who used to proudly advertise herself as "serving the new and unpublished author for over fifteen years!" Which sounds good until you think about it. Who wants an agent whose main claim to fame is that she represents unpublished authors. No! You want an agent who has many successful clients! (But not so many that they have no time for you. It can be a trade-off sometimes.)
Cover letters: Don't get me started! I have a two-hour rant on what NOT to put in your cover letter, based on years of reading through slush piles. What it all boils down to is: Keep it short, sane, and professional. Think of it as a job interview: you don't want to sound like a nut or an amateur. Mention any relevant credits, but do not ramble on at length about your life story, your artistic philosophy, or who should play the main character in the movie.
Note emphasis on relevant credits. This does not just apply to writing credits, but be careful about flaunting any other credentials. If you're writing a legal thriller, by all means mention that you're also a practicing lawyer. If you're writing a fantasy epic, on the other hand, the fact that you have a degree in neurosurgery doesn't really matter. Ditto for the fact that you raise prize-winning poodles. Save that stuff for your author bio, after you've already sold the book.
This may seem obvious, but, believe me, just keeping your cover letter short and business-like already gives you a huge edge over many other aspiring authors. There's really no reason why your cover letter needs to be more than three paragraphs long, tops.
Media tie-ins: Again, when it comes to tie-ins, most editors prefer to work with proven tie-in writers they know they can depend on. Dealing with Hollywood is tricky enough without throwing a new and untried writer into the mix. Best to make a name for yourself by selling short stories or whatever, then let it be known that you're interested in tie-in work . . . .
Hope this helps!