is it absolutely necessary that belle use pee pads or can you take her outside? outside and inside are VERY different, much more so than a pee pad and the floor. she probably thinks of the pads as her bedding now and that's why she won't pee on it.
i would suggest getting a crate. first of all, she will then think of THAT as her bed and will not go to the bathroom in it. then, when you take her out, you can take her outside (or to the pads if you have to) and try to get her to go. you should have a phrase (like "go potty) that you say everytime she is to go to the bathroom. then if she doesn't go in about 20 min, put her back in her crate. she will soon learn to not go in her crate cause it's her safe place and her bed.
please read the sticky on crate training here in the dog section before starting, though. it is vital that you introduce belle to it in the right way so that she likes the crate. she will of course, whine a little at first, but she will warm up to it and like it if you introduce her properly.
it sounds like you are doing everything right except letting her roam around after not going on the pads when you know she has to. this is where a crate would be helpful.
also, you are right in not letting her move while you are telling her to go potty. if you do start taking her outside, do NOT walk or play with her first. make her go potty first. and don't let her roam all over the play to look for a spot. just pick a spot and stay there. otherwise you'll end up having to take her for a walk everytime just to get her to go. they are smarter than you think!!
good job on what you are doing otherwise, though!
you are trying very hard it sounds like. and i think most everyone here knows how hard it can be to potty train a dog (or a kid for that matter!). haha! good luck! let us know how it goes.
(ps. i'm sure there is stuff i've left out. so someone else should fill in. sorry!! it's almost 5am here, though. haha!)