Norm, (another Norm?),
You can retain the rear cassett, or sprockets and still have several gears to choose from. The rear derailer,(I spell it in English because I'm in America and I'm not French) will act as a chain tensioner so there is no need to shorten the pedal chain. It will automatically take up the slack when you change sprockets with the rear derailer. By using the duel brake lever you'll have the clutch on one side, preferably the left, and both brakes, front and rear on the right along with the twist grip throttle. You'll loose one derailer control from the handlebars when you remove the front derailer. That should make your bars less cluttered and leave more room for those lights and horn you're required to have. Of course there's no requirement to mount those items on the bars. The headlight can be mounted to the steering tube or fork and the horn anywhere you can find to clamp it. Good luck and we're here for answers.
Also you need to take a close look at the shift/brake controls that you say are one piece. I have not seen this type but that doesn't mean they don't exist. Many of them appear to be one unit when in fact they are simply installed very close together with no seperation making them look that way. The Schwinn Jaguar with a seven speed twist grip shifter is an example. At first glance it looks as if the brake lever and the shifter are made together but they do came apart and can be moved and used seperately.
Tom