To answer your questions:
[O]ne offer was made after April 15th, and both deadlines were after April 15th: does the CGS still apply?
Yes, as is made clear in the webpage you linked to:
However, an acceptance given or left in force after April 15 commits the student not to accept another offer without first obtaining a written release from the institution to which a commitment has been made. Similarly, an offer by an institution after April 15 is conditional on presentation by the student of the written release from any previously accepted offer.
[I]t says I need a written release: how and who do I ask for this release?
You should ask whomever it was you've been corresponding with about graduate admissions. In particular, when you accepted an offer you probably signed a document written by a faculty member (e.g. the director of graduate studies in the department). This is the person you should contact as soon as possible.
As to the how: you should clearly and politely explain your situation, explain that you would like to get a written release to attend University B, and ask if that will be possible. In particular you should explain the part about B's offer coming with a 66% tuition refund rather than a 30% tuition refund. If that financial difference is significant or critical to you, say so, but don't exaggerate the financial hardship.
[H]ow likely is it to get such a release?
Well, I have not directly experienced this situation. But I am the Graduate Coordinator of my department (mathematics, UGA), so I can tell you what I would do: I would release the student from their commitment unless the student's reneging would damage the program and/or the department in a specific, serious way. (For instance, maybe if we enroll N-1 students instead of N, then we cannot run any courses for first year students. In my department this is a far-fetched scenario, but it gives you an idea of the amount of harm that could possibly be caused by a reneging student under exactly the wrong conditions.) On the one hand, I would understand that you would rather attend a significantly better program and pay much less tuition: so would I, and so would anyone. On the other hand, I would understand that your "commitment" to my program can't possibly be binding for more than one semester (indeed, more students drop out in their first semester than at any other one time), and that if you don't get a release you might be spending most of your time in my program planning your escape. Neither of those things would be good for either of us.
So far, I have not accepted B, but I really really want to.
As we established, you absolutely cannot accept B's offer while you have already committed to another offer. That is wholly unethical and dishonest. You should think a bit more and come to a state where you do not "really really want to" do something so unethical that it could jeopardize your future academic career.
The CGS resolution is not a legally binding document; it's a directive. So even if you do not get a written release from University A, it would be possible to inform them unconditionally that you are declining their offer. If you do that without informing B about it, then you risk making a bit of a mess later on when A and B each find out what happened (as is very likely; the academic world is not that large). So the minimally ethical action that doesn't clearly shoot yourself in the foot is to inform B that you will decline A's offer to accept theirs conditional only on this being acceptable to B. But as I said above, I strongly advise you to seek a written release from A.
I know there is a serious ethical issue with accepting and later declining, thus taking another student's spot, but I saw no other way because of the university A reply deadline.
You need to understand that A gave you the deadline it did because it is trying to act honorably in its dealings with a large number of students. So I deny the slight implication here that A did something wrong or put you in a particularly difficult position: it sounds like it played the game honorably. So you should seek to do the same. To be sure, getting a written release from A would be honorable behavior in this circumstance, so you shouldn't feel hesitant to ask for it.