A query should cover enough of the plot to get the agent interested enough in your book to request a full.
You can't assign a percentage to this. It's meaningless.
2) The book has 3 acts with tension and stakes building. This query is about the final conflict. Is that okay?
No. You want to tell me about what happens in the first act. If you're querying with what happens at the end of the book, it's sign that you don't have enough plot or enough at stake.
I have heard it said that a query should only cover 30% of the plot.