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Sei tu che non hai mai voluto prendere un impegno serio con me.
You use the constructions "it is you who ..." and "c'est toi qui ..." in English and French respectively with a third-person singular be verb. I wonder if in Italian the be verb in this expression always corresponds to the who in question? Is it incorrect to say:
È tu che non hai mai voluto prendere un impegno serio con me.
Yes, *È tu che non hai... is incorrect.
Each language has its own quirks, so do not try to find too precise parallels between two of them. Even two Romance languages, such as Italian and French, may differ in subtle ways, as in this case or, say, in
Si vedono le cose
versus
On voit les choses
(where Italian requires a plural verb, agreeing with the plural cose, while French uses a singular one). And many more cases.
Back to our sentence: decomposing it, you have a verb (sei) agreeing with it subject (tu), all followed by a relative clause introduced by che, whose verb agrees with its own subject (che) and so in the second person, since che refers to tu.
The case of Sei tu quello che... is different. It is a correct beginning for an Italian phrase, but has a different syntactical structure. The subject is again tu and the verb is again sei, but now they are followed by a predicate noun, or phrase. Simplify it by considering sentences like Sei tu il vincitore or Sei tu il mio migliore amico: those are simply emphatic ways to say Tu sei il vincitore etc.
Now, in your example, the predicative part is a whole phrase: quello che non ha mai voluto... Notice that the verb is in the third person here, since it agrees with the pronoun quello (or, more precisely, it agrees with the pronoun che that refers to quello). You could also say Sei tu la persona che non ha... and so on.
In this case, you might change the order of the words: Tu sei quello che non ha mai...: it would be a grammatical sentence. However, since you are apparently rebuking someone, the emphasis given by putting tu after the verb, and so stressing it (probably implying “it was not me, not anyone else, but you”), is quite appropriate.
On the other hand, Tu sei che non hai mai voluto... sounds quite unusual, hardly intelligible, and won't be found except, perhaps, as a poetic licence.
I confirm that È tu che... is incorrect, the only corect form is Sei tu che. The locution is the same as English, but its form is different. In Italian, the subject is the same as the dependent clause.
