A key word in 15 being “lawfully”.
See also Title 3, paragraph 2:
§ 2 Whenever any State has held an election for the purpose of choosing electors, and has failed to make a choice on the day prescribed by law, the electors may be appointed on a subsequent day in such a manner as the legislature of such State may direct.
The Constitution gives the duty of appointing electors to the legislatures of the states. They typically put this to a vote, but when the vote fails, as in the case of fraud, the legislature can direct the appointment some other way, such as picking who they think if the best choice or who would have won apart from fraud. (Ordinarily, the appointment of electors is done “by law”, that is, the state law. But, in the case of a failure such as fraud, they may “direct” the “manner” of their appointment.)
Paragraph 5 determines what is “conclusive” must also be lawful:
§ 5 If any State shall have provided, by laws enacted prior to the day fixed for the appointment of the electors, for its final determination of any controversy or contest concerning the appointment of all or any of the electors of such State, by judicial or other methods or procedures, and such determination shall have been made at least six days before the time fixed for the meeting of the electors, such determination made pursuant to such law so existing on said day, and made at least six days prior to said time of meeting of the electors, shall be conclusive, and shall govern in the counting of the electoral votes as provided in the Constitution, and as hereinafter regulated, so far as the ascertainment of the electors appointed by such State is concerned.
NOTE: in many cases the state law was NOT followed in the process of appointing electors. Therefore, if they were not lawfully appointed, they are not conclusive.
There is a big difference between trying to get the most accurate count of legal votes in a close election, and recognizing when massive amounts of votes have been cast illegally.
“””§ 2 Whenever any State has held an election for the purpose of choosing electors, and has failed to make a choice on the day prescribed by law, the electors may be appointed on a subsequent day in such a manner as the legislature of such State may direct.”””
I believe you are missing something in your analysis.
All 50 States and the DC certified before December 14th (the Safe Harbor Day) the votes by each candidate (slate of electors). Therefore, the legislatures do not come into play.