I'll answer as best I can. Keep in mind, I haven't grown lettuce so take my answer as you will.
1)My instinct says 3/4" should be enough. if you want to go with 1.1/2", 1" into the reservior will be plenty. vermiculite is a notorious wick.
2)I think either way will probably work. Personally I'd give them a rinse as dirt is more likely to harbor molds and what not than perlite/vermiculite.
3)The air pump in a raft system should be left on 24/7
4)In my experience, I have less bugs than planting in the soil. Less quantity that is. I've actually seen a larger variety of bugs. my first and only experience with horn worms was last year on the tomatoes. Also remember that since you're growing on an elevated surface, it's easier to spot and destroy anything that may get on them.

Thank You Sir, Really appreciated, plants going in this afternoon. I'd post a pic when I figure out a way to downsize My photos, LOL. Sorry if I'm a pain. Fred

Hello,

Just my $.02 worth; I start my lettuce seeds in Rapid Rooter, no need to transplant, rinse, clean off dirt. I put them directly in 3" net pots at the 3 week point I use clay pellets to hold them in place. It's very easy

Here is a link that might be useful: 26 Day Old Lettuce

On insects: I grow lots of lettuce. Get ready for aphids! They are brought in by ants, so if you already see ants, get rid of 'em!

The aphids can be controlled spraying the plants with mild soapy water with chrysanthemum juice mixed in. It won't hurt the taste of the lettuce, but it will kill the aphids.

Check with your local hydro store, they probably sell a product like this in their inventory.