1.Your current lighting would be well suited for a non carbon tank. I agree with Creighton on the AH lighting. I run 55w PC over 30 gals using excel with no problems. The retro kit can fit in your current factory hood, call AH Supply and they will help you.
Well since I plan on adding more plants (obviously) I feel like my current lighting setup wont provide the results that I'm looking for
2.Airstones are completely fine in an Excel setup. There is no issue with CO2 in an Excel tank ie. the Excel is the carbon source.
I'm waiting on using Excel until I get a lot more plants in my tank 
3. Again, with my current lighting on a 30 gal(55W PC) Excel tank I am able to grow carpet plants that cover most of the bottom. Substrate is Eco-Complete.
4. Crayfish
IMO i wouldn't have them. 4" could uproot a lot.
So far i haven't had any problems with him trying to uproot anything
I advise against a glass cover. Might start OK but in time will get water spots and reduce light. Then one more thing to clean IMHO. All my tanks are open top, have had little to none in the way of jumpers.
The T5's might be OK. I tried once, but wasn't my favorite. I lean toward the PC with high quality reflector.
What didn't you like about T5's?
Also, you need waaay more bio mass before using Excel and more light as Creighton said. And of course dosing will need to be addressed also
See my answer for #2 