In a scenario where a Trump incited mob executes Senators while the police and national guard do nothing is tantamount to a military coup. The replacements would have no reason to believe that their lives wouldn't also be in danger if they didn't certify Trump as the winner of the 2020 elections. At that point Trump has effectively side stepped the Constitution and usurped power through violence - which is literally the end of the republic.
But that’s not even close to what happened. The national guard doing nothing would require buy-in from the entire military chain of command, which has never been very supportive of Trump.
If the Sentors and/or the VP were taken as hostages, or worse killed, it would mean national guard was either complicit or incompetent. However, as we know, the Secret Service sadly had to act with deadly force and managed to secure the legislature.
Nope, all of the states would appoint new representatives. There are processes for all of these scenarios.
The senate and house are not necessary for the day to day operation if the republic. They aren’t even there all of the time. Killing a bunch of politicians is not a threat to the republic.