I think it went well for your first attempt. Grommit took the food from you and you managed to get the knot out of his hair. He probably did not like his hair being cut as he is not used to it but he is still trusting you as he took the food. Do not be hard on yourself. You had to get the knot out of his hair for his well being. I think you did the right thing in putting Grommit back in his cage when he went into his tube. Sounds like he is learning how to communicate to you and you are learning what his body language means.
Maybe Grommit would feel more comfortable if his playpen is a little more cluttered? That way he has more hidey places. Just an idea and I may be wrong but thought I would mention it.