a) Weight loss inevitably slows down REGARDLESS when you diet and actually, you can't really lose more than 1.5lb of fat a week without exercise, and here's why: a 1.5lb of fat loss = 750 calorie deficit a day. I doubt you have any more than that on your WW plan. To lose 3lbs a week you'd need a 1500 calorie deficit = ie 500 calorie a day starvation diet.
I would bet that your weight loss would have slowed down around now even if you hadn't done shred.
b) shred will help preserve your muscle mass as you lose weight (as will eating plenty of lean protein) - do you want "weight" loss so much that you don't care if you are losing muscle rather than fat?
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c) As well as helping to preserve your muscle, shred WILL help with your weight loss as it burns 200-300 calories, so if you do every day that's an extra half pound a week. Plus resistance training has an afterburn AND increased muscle mass increases metabolic rate.
d) Realistically you can lose 1.5lb of fat a week (if you can mantain a 750 deficit a day), so around half stone a month, so if you have until say mid February, that's 6 months = 3 stone, so totally do-able. BTW maintaining 750 calorie a day deficit quite hard as you get closer to target, would be less than ww recommends eating I would have thought.
e) If your weight loss slows down (lower than 1.5lb a week), look at your diet, I got over weight loss plateau by changing WHAT I ate, I did dukan for a bit but now I eat "clean" as far as possible - lean protein at each meal, stacks of veggies, some fruit, healthy fat each day and unprocessed carbs, esp after exercise. But see above, 750 calorie a day deficit is hard - assuming maintenance weight is 1800 calories, you need to be eating only 1200 and doing something like shred each day.