Not a native speaker, but the sentence sounds to me like a comment to "Han gav mig ikke sit nummer, (må jeg bede om DIT nummer?)". Is the stress correct even in that case?

We can't fix this atm :(

No, you can't omit "for," since that sounds like you are talking to the number. You ask somebody for something.

(There are a few special cases where this rule doesn't apply, though not too many spring to mind. For example, you can ask forgiveness -- though you can also ask for forgiveness. But you can't ask tolerance; there the "for" is required.)

Jeg tror at "Deres" er meget mere høfligt og formalt en "dit", her.

"Deres" is very formal. You will typically only use it for royalty, the elderly, your boss or your teacher.