Tense/Verb Quagmire

Both sound fine to me, so I looked it up. According to this, what you're talking about is "future in the past." It says there's a slight difference between them ('going to' used more for planning; 'would' more for promise or voluntary action) but both are used for predictions about the future. Hope that helps.

"Would" is conditional, while "going to" is future. The implications are different; "would" implies that the decision has not been made, while "going to" implies that the decision has been made.

Both are fine and mean approximately the same. These are rhetorical questions, so to simplify the issue let's convert them to their implied statements.

I would not ditch him for someone younger.
I’m not going to ditch him for someone younger.

The former is a stronger statement. It implies that under no circumstances would you ditch him for someone younger. The latter says only that in the actual future, you won't ditch him for someone younger. Maybe no one younger is available, but you would if they were.

Also note that "I will not ditch him for someone younger," means exactly the same as the latter. Changing "will" to "would" makes it hypothetical.

Although there is a subtle difference between "would" and "going to" in this case, the meaning in both sentences is the same and clear. It's just like:

Do you honestly think I wouldn't miss you if you leave?

or

Do you honestly think I'm not going to miss you if you leave?

Same meaning.