Paul J, I don't think this is where Lotus are heading with this car. They are aiming at a more mature audience, so the car is designed to be more luxurious and heavy. I am guessing a light weight stripped out version will be inevitable at some point aka 911 GT3, but that isn't the target audience.
The reason there will be a gap in time until the drop top will be more down to marketing and keeping the image of the car "fresh". They could probably produce the convertible sooner, as you have suggested but this will possibly over expose the car in the market, shortening its life cycle. Don't forget they want to make it last for eight years, an long time in car terms.
It's all about the twisties....