The fact they even agreed to trade him simply shows they understand he no longer wants to play for them. So in that respect Taylor has some power to force a possible trade.
Yet he is still under contract to the Colts and even if he wants to play for the Dolphins, the Colts are going to do what is best for them if they do trade him. So when it comes to trading him to the team that offers them the best deal for Taylor, the Colts hold all the power.
Taylor can obviously decide to sit out the year and not get paid. I doubt the Colts would put him on IR because that would mean he would be paid the entire season for not playing.
I agree with you that this is the year they can get the greatest trade value for him. That is also why I think him coming to the Dolphins is less certain. Because the Colts need to get as much value for him while they can and I’m not sure the Dolphins are the team that can offer them the best deal for Taylor. I guess we just have to wait and see how this plays out.