Is it possible to complete a blade's level 3 Affinity Chart reward before completing the level 2 reward?
While my current save is not in a really good spot to test this, but I'm going with No. While reviewing Myrtha's affinity chart, I have her level 4 nodes unlocked. As you can see in the screenshots below, she has a handful for level 2 and 3 not completed. I know for a fact I've used Lightning Buster and Photo Edge a numerous amount of times after unlocking these nodes, but yet they do not any progression towards completion.
Will progress towards level 3 count if I haven't completed level 2?
The answer to this one is that it depends on the node. As shown above, progression towards level 3 nodes does not count if level 2 is not completed. However, there are some nodes that have "Previous instances count toward total" as sub-text