There is no time limit placed on “fails to qualify” in the 20th amendment, section 3. In fact, section 3 does not even mention the electors.
Sure there is. It's right here, "at the time fixed for the beginning of the term of the President..." Well, at the time fixed for the beginning of the term for President Obama was held to be eligible. If it turns out that was wrong then he gets removed and Biden is president. Just like the 25th Amendment says.