It's not necessary, but both chains can't coexist unless one changes certain network parameters, such as port number.
Yes I remember that 50 coin's chain fork client somehow detected blocks from the bitcoin original chain and that made it stop to work
It does not matter it is MAX_BLOCK_SIZE or nSubsidy, as long as they can be changed through a software upgrade, there is a POSSIBILITY that a future software upgrade will change some bitcoin characters. This possibility is the biggest risk and uncertainty of bitcoin
I concede that the possibility exists. That's different than claiming that it's a non-trivial risk.
If bitcoin has gained mainstream acceptance, and some powerful entity like CIA decided to change these rules, it will be much easier than when these rules were hard coded in the blockchain itself. They just need to ask a software company to bake the US government version of bitcoin client and push it into major exchanges
At least they will debate why these rules can not be changed, that will take lots of macro economic related discussion