Can you lose weight in a healthy manner, without exercising?

Yes, it’s certainly possible to lose weight without exercising. The best diet for weight loss AND health/energy is easily a dumbed down version of The Zone described in a book with an unfortunately sensationalistic title:

http://www.amazon.com/exec/obidos/tg/detail/-/0345419987/qid=1086177589/sr=8-1/ref=sr_8_xs_ap_i1_xgl14/104-4543138-5494304?v=glance&s=books&n=507846

Don’t be fooled by the title though. The basic premise is to stabilize your blood sugar to avoid the insulin spikes from highly gycemic carbs. The insulin converts blood sugar into fat, and the amounts released from cheap carbs will be overkill, lowering your blood sugar too far, making you hungry again. It’s the worst cycle possible for weight loss. The Zone is too detailed and obsessive to be practical.

As far as Weight Watchers, I don’t recommend it. Sure, if you stick to the calorie counts you will lose weight, but doing so can be tough. With blood sugar stability you aren’t hungry in the first place.

Yes, you can lose weight without exercising. All you have to do is to eat less food and you’ll lose weight.

However, one problem with being sedentary is that your caloric requirements are so low that you might not eat enough food to get all your nutrition. You might get enough calories, but you might not get enough vitamins, minerals, etc. If you’re active and eating 2500 calories a day, it’s easier to get the nutrition you need. But if you’re only eating 1000 calories a day, you’ll have to be very careful to eat foods which are high in nutrition for the amount of calories they have.