1. Until someone discovers how to obtain it, nothing can be said for sure.
2. Theoretically, yes, achievements are separated by their grade of difficulty. However, there are Grade 1 achievements that are more difficult to obtain and should be at a higher grade of difficulty. The Fearless achievement is an example. It's hard to understand CipSoft's criteria, so it might be something simple, or something that could take a long time.
3. It's usually related to the update in which it was implemented, but there's no rule about it. If CipSoft wants to implement new achievements related to old content, they can. In update 8.62, for example, several achievements related to older content were added.
And yes, they can implement achievements even before they can be obtained. The King's Council achievement, for example, was implemented in update 12.70, but only became obtainable in update 15.20.7a5cc9.