Saying that XT is an altcoin is factually incorrect, this is also a false narrative. Bitcoin XT is compatible with the Bitcoin blockchain. The fork will only be triggered once a majority of mining power supports it, which means that post fork Bitcoin XT will also be compatible with the largest surviving Bitcoin blockchain. There is a fundamental difference between a blockchain fork and an altcoin. It would be wrong to equate these two separate phenomena.
That entirely depends on what the actual definition of altcoin is, this is an evolving space and definitions are being settled.
If Feathercoin is an altcoin then so would be XT.
If we define an altcoin as being a blockchain that is incompatible with the main Bitcoin blockchain then according to this definition Bitcoin XT is not a altcoin.