'Whom' is more formal than 'who'. You'll rarely hear it spoken in American English. Also, 'whom' is usually used with a preposition like: to whom, for whom, about whom, of whom, etc and is the OBJECT of the sentence. 'Who', on the other hand, is the SUBJECT. In your example, 'to' is written at the end of the sentence, but the meaning is still 'to whom'. Therefore, you may use 'whom', although in informal spoken English 'who' is more commonly used. (If you're taking an exam to test your English language ability, use 'whom' as the object of the sentence.) https://www.grammarly.com/blog/who-vs-whom-its-not-as-complicated-as-you-might-think/

Yes, the second question is correct:

Do you know the woman whom Jack is talking about?

If you can replace the word with “he”' or “'she,” use who. If you can replace it with “him” or “her,” use whom.

Although, who is usually more common in everyday British English conversation.

Very interesting "Do you know the woman with whom Jack is talking?" "Do you know the woman who Jack is talking to?"

This is grammatically correct, but awkward. Colloquially (in the US, at least), we would say "Do you know the woman Jack is talking to?"