"¿Por qué no habló ella ayer?"

Yes to both :)

slowmotionfro- Anyway, hablo, present tense and habló past tense, don't have the stress on the same vowel. So during the conversation you'll hear if it's present or past. For present the stress is on A and for past, the stress is on O, with the written accent.

You don't have to say "ella" or "él" when it's clear who you're talking about.

That would mean something different. It would be something like "Why did she not speak to (him/her/you) yesterday?"

por qué means "why?" (used in questions)

porque means "because" (used in answers)

joe- 2 mistakes : porqué, because it's an interrogation and no hablo without accent is nonsense in your sentence : why I don't speak yesterday, that's what you're saying. That's why accents are so important, habló is for él/ella/usted

Ella is the subject here, doing the speaking. Only direct objects get a personal a.

Those sentences have different meanings. You'd say "Why didn't she say something yesterday?" if she had some kind of issue yesterday that she did not talk about. I'd translate this as "¿Por qué no habló ella nada ayer?"

The original sentence, however, indicates that she didn't speak at all that day, maybe because of a throat infection. "Why did she not speak/talk yesterday?" Or "Why did she not call yesterday?" in context of phone calls.