1. Well for Archer he became a Counter Guardian in one universe were he wasn't summoned into the 5th HF War. That would mean that Shirou may have seen Ajax in action at that point, and later on after the war still had the ideals that his father once had. It just takes one universe for a Counter Guardian to appear from my guess of course. After he became a Counter Guardian I would assume that things during the 5th HF would change dramatically, hence why there are 3 various ends on how the war ended.
2. For someone to become a Counter Guardian it would mean that they would establish a contract with the World itself. Although not everyone has the ability to become a Counter Guardian even though they have the abilities to become one.
- Finding my way through this garden is rather unbearable, but if it is to keep the one I adore company I will endure it for her sake, and her sake alone.