Sequence of Tenses \ informal situation
"I will come" — she said.
"She said she would come" — I said.

This is how it grammatically should be. But I heard that sometimes in an informal situation you can just say "She said she will come", is this correct? How often do people usually use the first and the latter variant? Percentage?

Are there any other cases like that? Can you give some more examples (with and without will\would).

Question

Sequence of Tenses \ informal situation
"I will come" — she said.
"She said she would come" — I said.

This is how it grammatically should be. But I heard that sometimes in an informal situation you can just say "She said she will come", is this correct? How often do people usually use the first and the latter variant? Percentage?

Are there any other cases like that? Can you give some more examples (with and without will\would).

Answer

She said she would come = reporting what she said
She said she'll come = quoting what she said

Geddit, pal?

Answer

This is known as reported speech and the transition into a future tense is something we get from German, who still use their subjunctive to report speech ("Er hat gesagt, dass er wohl sei") but in English it remains in a possibility to relax the sentence into the future.

"She said she's coming", "She said she'll come" and "She said she'd come" (either 'she had' or 'she would' as "come" is both the infinitive and past participle").

I would suggest that it is *slightly* more informal, but you shouldn't feel wrong using it in written or even quite formal speech. Other examples I can think of:
Note: the construction is most often pres. + future, but the tenses can shift around.
"He said he'll think about it."
"They think they'll be happy there." (note the emission of "that" in casual English)