Well, that sounds stressful, but not as bad as I was imagining!
My .02 - your DD needs more context about this touch-feely guy with the bike, and how special her relationship with him is. She needs to know that he's her birthfather, planted the seed that made her, etc. etc., and that his love for her isn't creepy or inappropriate or unusual. He's NOT your friend or your neighbor, and behavior that would righteously freak her out if the man next door tried it is behavior she can hopefully learn to be comfortable with from her dad. It's too bad dad can't ease into a bit more - but he didn't choose his limitations in life, and if there's going to be contact, then to some extent the traditionally-abled people in the triad need to accept him as he is.
Also, let her keep the training wheels for a little while - they were part of the special present from her dad. If she happens to love them (which I doubt, as she's used to the smooth ride of a balance bike), even then I would not sweat it. I agree that training wheels aren't ideal, but my 5-year-old enjoys hers and I' have every confidence that she won't be using them when she's 12.
I agree with you on the language issue. If I were to adopt my current foster child, I would not encourage/allow people to have any kind of conversation with him that excluded me. He's too little to know when people are playing him or lying to him, and it's my job to be able to run interference for him with anybody who might try. This is my first big issue for the GAL when we finally get one - to make sure that visitation isn't a two-hour-long private conversation where the monolingual social worker is just siting there technically "supervising," but really, letting mom say whatever crazy lies occur to her. (Tough week with Zeke's case, as though you couldn't tell from my bitter rant.) Having an interpreter attend the visitation is a solution for now - long term, the solution is you and DD gaining basic fluency in Spanish.
It's really too bad that your DD wasn't able to have this visitation from babyhood on. I know that you were open to that, and I'm sorry that other factors precluded it until now. There's a lot of hard work to be done to fill in that gap in the relationship. For subsequent visits, maybe you could just bring DD and have your other kids stay with a friend or something? So you can be 100% focused on helping her out and she can enjoy her gifts without having to deal with sibling jealousy immediately?
You're a great mom, Polliwog.