Nearly all nations in the world already "use Bitcoin" in the sense that they are capable of doing so for any transaction that they want to do and the currency that they use is agreed between the two parties doing the transaction. It'll never become legal tender because of the nature of government regulation, and it'll never replace legal tender for the same reason.
I think not. Just some country. using and capable of using is very different. The first one had already committed an action while the latter one just stating the capability and neither commit any action towards the subject. Honestly your words are confusing, Japan already declared Bitcoin as legal tender so that contradicts your stand as you were stating the countries in this world as a whole in your argument.
Even if not all countries use Bitcoin, but rather acknowledge Bitcoin, it will increase in price exponentially since adoption will increase exponentially causing the demand to pile up which can cause people to panic buy outbidding other buyer just to have a piece of Bitcoin.