No, Harris would be replaced first. Then Biden would leave. Pelosi would not become President under that scenario. 25th Amendment governs:
Section 2. Whenever there is a vacancy in the office of the Vice President, the President shall nominate a Vice President who shall take office upon confirmation by a majority vote of both Houses of Congress.
Remember how Ford became President?
I do not know about that, i am speaking to the line into the oval office. The dems are in a loose, loose situation.