Welcome to the wonderful world of Tri!
I started to train about May/June last year as I thought I was getting a bit too unfit, and I am loving the training.
In response to your questions, and based on advice I have been given:
1. You won’t lose weight overnight. As it seems you are aware it all depends on your body type. You will also be developing more muscle in the place of fat. You will soon see that there are all shapes and sizes of people involved in multisports so in your first year I wouldn’t get too hung up on weight and concentrate on eating high quality carbs and low in saturated fats. By virtue of training and eating right you will naturally lose the weight over time. I found having one day a week where I could eat whatever was really great – something to look forward to.
2. I am doing Blenheim too, I believe there are training days being run by the organisers for open water swimming so may be try and get on one of those? How about entering some stand alone open water swimming competitions so you don’t have to worry about transition or then getting out and cycling?
3. Personally, I have been doing other events such as Aquathlons and Duathlons. I have found it has helped me to work on different aspects of transition, such as continuing to work hard after a swim or the changeover bike to run. Brick sessions are good too, this is where you say go for a bike ride and have your trainers ready to do a mock transition before running round the block for say 10-15 mins.
With regards tumble turns (or the American ‘flip turn’ if you see some on Youtube), there is a mix. The more experienced swimmers will do them, but if you are out of practice it can cause you more problems that it solves. I would say go with whatever you are comfortable with, but in the longer term will probably be beneficial to learn how to tumble turn.
Good luck with training and your first venture into competitions.