This is a detailed and subtle point of grammar (and, to be honest, there are other more significant problems in syntax).
Firstly - what are you talking about? The stranding of of the preposition "to" in the sentence
Who did you mention to?
But this already wrong, since mention needs a direct object. It seems you want to have a "that clause" as the object. This is a content clause; it's not a relative clause
Who did you mention that I was too late to?
The error in (1) is not stranding the preposition, but the placement of the content clause.
(2) is a reasonable spoken expression... The speaker doesn't want the long interruption of the content clause, so uses a pronoun "Who did you mention it to?" That is correct, but then the speaker feels that the reference of the pronoun isn't very clear, so supplies it in a sentence fragment. "... that I was too late". The content clause doesn't have a syntactic role in the sentence. It is a fragment. That kind of grammar is possible in speech, but should be avoided in writing. In writing you should use sentences and avoid fragments. As a learner you will find that you use fragments by mistake in speech and it is okay, but it isn't something you should practice doing.
(3) has the same problem as (1), the word "mention" needs an object. But you could fix it in a different way - using a different verb, one that is intransitive:
Who did you speak to about the honeymoon?
So key points are (1) Mention is usually transitive and should have a direct object. (2) Some "that" clauses are not relative clause, they are content clauses.