Threshold of what? Of nodes? How can you know the percent of nodes that agree with the new rule?
Percentage of blocks.
The users are those who follow the consensus rules, the ones Satoshi had chosen. A change on a consensus rule confuses the people of what is the real bitcoin.
I disagree. I think users should have the freedom to choose what is defined as Bitcoin. You don't use Bitcoin just because Satoshi said so; you use Bitcoin because you stand with the rules that governs it (21mil coins, non-reversibility, etc). Having a fork such as the one that you have described would definitely not be unpopular but people can just continue on the fork that doesn't implement the fix like how Bitcoin Cash is formed.
There will be ways to prevent attacks from quantum computers. The question is how they'll force every node to accept them.
You don't have to. If you don't want to implement the fix or to not burn the old coins, then an altcoin would be forked from that, just like how ETH and ETC came about.