Interesting thread. It sounds like your previous experience with a sales rep was not great. There is no problem with your weight. 235lbs is not that heavy.
There is no reason to avoid carbon, aluminum, steel or ti. They are all fine materials. I would recommend that you go try out entry level bikes as another poster stated. Ride it for a year or two and then, if you like the sport, you will know exactly what you want. It is nearly impossible for a new rider to know exactly what they want or need until they get into the sport and ride for awhile.
None of your questions are silly at all and you should not feel that way. We all have had those same issues at some point.
Every LBS has a different way of doing things when it comes to test rides. I personally think that test rides are a little over rated as it is hard to really tell much by riding for such a short duration. My first bike, the shop sales rep. took me out and rode for about 10 miles. The other bikes that I looked at the shops just let me ride around the parking lot.
Visit a few shops and ride a few bikes. Ask questions and see what shop you feel comfortable with. All the bikes are pretty good. Bikes within the same price range are typically very similar as far as quality goes. Figure out your price range and stick to that budget. Most places will try to sell you a model up from that. It is natural but you can spend a lot more than anticipated if you are not careful.