I think, L, that while your input is important, especially to share your feelings with everyone involved, the relationship between T and D should evolve at a pace that's set by T and D. So it's probably more important that they talk about it together rather than you setting the pace.
You might think it's going too fast for D and you might be right, but it's D's place to tell T that, unless D asked you to do it for her. Otherwise, as much as you want to help, you might actually not help at all.
It's good that T and you talked together, but it might be better if T, D and you all talked together. Or you could talk with D directly, see how it goes for her. I personally wouldn't be very happy if my boyfriend's wife (if he had one) was telling him to slow down... Even if I did want him to slow down! I wouldn't want to be left out of the decision of whether to slow down or not in my own relationship!
So, I'd say it's either between T and D, or you can be involved too, but with both T and D, either together or separately. Otherwise, yes, it might appear as you trying to control the way their relationship evolves, whether that's your intent or not.