Well Galadriel doesn't see it as prison. Middle Earth is where she believes Sauron is at and as long as he is there he is a threat to even Valinor. So her mission is to defeat Sauron recalling what happened to her brother. So being sent back to Valinor would go against her desires.
All of this just boils down to writing. If they just spent the first episode on that one story line and fleshed it out, then it may make sense. But I literally believe that their goal was to get her on that raft and this is was the only way that they could accomplish it. And all of those other things are just written that way to make it happen and accomplish the plot points they wanted to accomplish. Numenor is an island away from Middle Earth. We don't know what kind of relationship the Elves have with them at this point in time in this series. So they had to figure some way to get her there outside 'official' channels. To me it is just obviously bad writing, especially since all of these things so egregiously contradict the lore of the 2nd age.