Your information regarding state law is irrelevant.
US Senator is a federal office and thus is subject to federal laws.
Mississippi’s election laws in relation to federal offices don’t matter and they know it. Federal case law is solidly against them in this case.
Not according to what you posted.
According to what you previously posted, states can make their own laws about ballot access (which is what independent registration and write in candidacies are all about). The state laws matter because they exist. In order to challenge them, to either overthrow them, or to get a variance because of fraud, requires a court case and a ruling.
The laws are there, so they matter. Your opinion about them is actually irrelevant. So I revert to my original statement. Mississippi law prohibits write in candidates, (at least in the general election).