Is it possible to shorten these kind of sentence structures?
The title of the event, that will be tomorrow is XY.
Is any of these correct?
The title of tomorrow's event is XY.
The title of the event tomorrow is XY.
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Your first sentence is, at best, awkward. As stangdon says in the comments, you should remove the comma. Or use a non-restrictive clause: "The title of the event, which will be tomorrow, is XY."
The most natural way to say this is, "The title of tomorrow's event is XY." Or as Fumble Fingers says in the comments, specifying "title" is probably unnecessary, and you would just say, "Tomorrow's event is XY" or "XY is tomorrow". You probably only need to specify that this is a title is there is some possibility of confusion. Like, "The title of tomorrow's event is 'Friday'." Not necessarily that tomorrow is a Friday, but that the title of the event that will be held tomorrow is "Friday". I suppose there could also be some technical distinction you need to make, perhaps in the program book the event has a "title" and it also has a "subject" and you need to make clear which you're talking about.
You are wrong in classifying today, tomorrow, yesterday etc as temporal adverb. They are best classified as Pronoun, to be precise, Deictic Temporal Pronoun. You unconsciously gave the reason. One simple reason of classifying them not as an adverb or preposition is that they have genitive inflection - today's, tomorrow's, yesterday's etc.
So it's right to write -
1. The title of tomorrow's event is XYZ.
2, The title of the event tomorrow is XYZ.
3. The title of the event that will be tomorrow is XYZ.
In all these sentences above tomorrow is a Pronoun. But they don't have the same function in all these sentences.
In sentence #1, the genitive pronoun functions as a determiner. In sentence #2, the pronoun acts as a temporal adjunct. In sentence #3, it's a complement, that expresses "time".