Do the 240 days on I-539's overwrite the request date (in my case 180 days) on the I-539 if USCIS doesn't respond?

I believe that the 240 days to which you refer are for extended work authorization of certain work-related visas when the petitions have not yet been adjudicated by the ending time of the visa status. That is usually associated with form I-129 rather than I-539, the latter of which is used for other nonworking visa extensions. If you applied for a B1 or B2 extension during the time of Covid, and wish to remain in the best state of legality, you would likely be better off filing for another extension prior to the expiration date of the time that you have requested even if no adjudication has yet been made. Due to the limitations of the Lawyers.com Forums, Alan Lee, Esq.'s (the "Firm") participation in responding to questions posted herein does not constitute legal advice, nor legal representation of the person or entity posting a question. No Attorney/Client relationship is or shall be construed to be created hereby. The information provided herein by the Firm is general, and requires that the poster obtain specific legal advice from an attorney. The poster shall not rely upon the information provided herein as legal advice nor as the basis for making any decisions of legal consequence.