I believe I do have a timeline of events that you might find enlightening.
Gally phones Newt to ask him to do her a “colossal” favour by filling in for her at a Midnight & Noone performance in Leeds. She tells him that he will need to play the cello, but will only have one small bit of singing in the Albert Small song.
Despite Newt’s protests, Gally insists that he must do this for her. Gally tells him that he can borrow her clothes, but he will need to buy himself a top hat. (Presumably, this is because his head would be too large for Gally’s top hat.)
Newt is worried that the audience will be disappointed if they are expecting a male impersonator but get an actual man. Gally dismisses this by saying that they will just think he is frightfully good.
Gally warns her brother, Newt, that she has a “super colossal” favour to ask him.
Through cryptic references to Susanna’s pretend husband, Major Noone, Gally asks if Newt would father a child with Susanna, which Gally and Susanna could then raise together.
Newt is reluctant to do so. In his life generally, Newt has no interest in the required activity and never intends to try it. He remarks that there are other ways of achieving the same effect.
At this point, Gally begins to speak more plainly. She had particularly hoped that Newt would do this for them, so that the kid would be the closest possible thing to being Gally’s own child.
This convinces Newt and they agree to the plan. He asks to be part of the child’s life in the role of an uncle.
Gally begins to explain the practicalities of the arrangement by asking if Newt knows Crewe.