Q: If Jesus fused, why didn't he go up in flames? Is it possible to fuse and still remain on the Earth?
A: I was happy to answer your question, and I can see why there might still be some confusion. We learn in The Urantia Book that mortals who fuse with their Adjusters while still on the planet go up in a flash of fire. Obviously, Jesus did not experience this.
I did some searches of The Urantia Book, and I was not able to find a definitive answer to this question, but maybe we can reason it out (being mindful that it may not be something that can be entirely understood.)
What occurs to me right away is the obvious problem that would have developed if he had fused...at the time of Jesus' baptism, his ministry was at its very beginning stages; to have him disappear from Earth at this stage would certainly have been counter-productive to his completing his mission of revealing the Father and establishing the Kingdom.
The passage that I cited above has this statement:
(136:2.1) "He stood in the Jordan that day a perfected mortal of the evolutionary worlds of time and space. Perfect synchrony and full communication had become established between the mortal mind of Jesus and the indwelling spirit Adjuster, the divine gift of his Father in Paradise."
Since Jesus was also Divine—already of the Father himself—there likely was no need for him to fuse with his Adjuster as other humans do. Instead he was given the vision of himself as he really was:
"This ceremony was the final act of his purely human life on Urantia..."
Jesus, while human, also was God. This makes him a totally unique person. It seems to me to have been unnecessary for him to fuse in the usual way, as the partnership of his mortal self with his Adjuster was already perfected that day in the Jordan. As well, as noted above, his mission would have been effectively ended had he disappeared in the flash of fire. Since he now knew of a certainty who he really was, he was then able to partner quite effectively with his Personalized Adjuster for the remainder of his earth life.
...the Son of Man saw the vision, presented by the now Personalized Adjuster, of himself as a Son of God as he was before he came to earth in the likeness of mortal flesh, and as he would be when the incarnated life should be finished. This heavenly vision was seen only by Jesus.
Throughout the remainder of Jesus' earth life this Personalized Adjuster was associated with him in all his labors; Jesus was in constant communion with this exalted Adjuster.
Did this Adjuster indwell the mind of Jesus, as is true of us? It sounds as if it did not, but remained somewhat separate from him, but "associated" and in "constant communion" nonetheless.
As to the second part of your question:
I can think of no good reason for a person to remain, even if s/he could, once fusion has occurred. But aside from that, we read that:
so there would be no body left in any event. And once fusion has taken place, the entire purpose for a mortal life has been fulfilled, so no reason to go further in that state of being.
...fusion during physical life instantly consumes the material body... (110:7.2) )
I hope that this reply has been useful to you. Thanks for writing back, and please feel free to write again, anytime...