I have to admit that I haven't experimented a lot with REST, so maybe some REST-experts here on HN will help me out answering some of these questions or give feedback regarding the answers that will follow in the next paragraphs :)

regarding question 1: I think it's all a question of how your method behaves -- or how it is implemented. if your method requires JSON, then you'll have to feed it JSON. if your method requires 2 parameters, then you'll have to give it 2 parameters (e.g. myservice.com/?param1=123&param2=456).

regarding question 2: no, it's definitely not a problem, until the library is not maintained anymore or god knows what else. well, yes, it's more intuitive to write a plain simple function then to construct a very complicated request string as you mentioned.

as I already stated above, I don't have enough experience with REST at the moment to give you a more precise response, so I hope this one suffices. I also hope that somebody here on HN with more experience in the REST field will answer your question more precisely, so I can learn a thing or two, too :)