lots of questions here and no responses....i'm guessing thats because your questions are not necessarily simple, and also because some of them could have been answered by doing some searching.
nevertheless, i will give you a short version:
1. 48x39 is standard for cross gearing. using the DA as you have it could very well put you at a disadvantage.
2. a single 42 ring is indeed a nice way to go, but setting it up is not always that easy. you will need to consider your chainline, bb, and chain guard(s) and/or a chain watcher to avoid dropping your chain.
3. tyre pressure is a science in itself. the psi you can run in a clincher will be determined by the course conditions, weather, terrain, and your weight. unless you are a clydesdale, 80 psi is almost certainly WAY too high. the key in cross is to run them as low as you can get away with (and for clinchers, this is higher than tubs because of the risk of pinch flatting).
4. as for in-line levers. this is a matter of preference. i happen to like them because i come from a mtb background, so i prefer to be on the tops when things are technical. the salsa's weigh nothing, so having them cant really hurt you.