Having thought about it some more, I think there are some holes in this theory.
While the script does draw attention that Rider would only have recovered enough by nightfall, I wouldn't take it to mean literally that they had to wait until it is dark to attack. If Rider has a plan, he would be able to put it into action by evening. The point of the scene was that Waver was going to be resting in the middle of nowhere for most of the day, nothing more.
Furthermore, Rider had already made it clear that they should start with Saber, and while we don't know for certain if Rider knows the location of their hideout, I wouldn't rule it out either. Remember that out of all the Servants, Rider is the one who has been known to fly around the city on his own initiative, and being a conqueror he is very familiar with the need to determine where the enemy is in advance, and strike when you have the chance. The dialogue back in ep9 made it obvious that Rider is in favor of attacking head on, which isn't very different here.
But even with all these points, it is still possible that the attack was forced by a third party. What I feel doesn't make sense though, is how that is likely. Waver might not be an experienced mage, but he's not incompetent. Like all mages, he can sense the presence of other Servants, so it is unlikely that Gilgamesh would be able to get anywhere near the area without Rider materializing in defense. Kirei approaching on his own is possible, but Rider would also be able to materialize and stop him. It would be suicide for Kirei to make such a rash move against such a strong Servant.
One thing we should note is that the end of the scene with Waver concludes with him saying "do whatever you want." This is the act of consent indicates to me that he is giving Rider permission to formulate his own plan of action, and what follows is likely to be 100% Rider, and not something Waver cooked up. For all we know, Waver could still be sleeping.
Furthermore, Rider had already made it clear that they should start with Saber, and while we don't know for certain if Rider knows the location of their hideout, I wouldn't rule it out either. Remember that out of all the Servants, Rider is the one who has been known to fly around the city on his own initiative, and being a conqueror he is very familiar with the need to determine where the enemy is in advance, and strike when you have the chance. The dialogue back in ep9 made it obvious that Rider is in favor of attacking head on, which isn't very different here.
But even with all these points, it is still possible that the attack was forced by a third party. What I feel doesn't make sense though, is how that is likely. Waver might not be an experienced mage, but he's not incompetent. Like all mages, he can sense the presence of other Servants, so it is unlikely that Gilgamesh would be able to get anywhere near the area without Rider materializing in defense. Kirei approaching on his own is possible, but Rider would also be able to materialize and stop him. It would be suicide for Kirei to make such a rash move against such a strong Servant.
One thing we should note is that the end of the scene with Waver concludes with him saying "do whatever you want." This is the act of consent indicates to me that he is giving Rider permission to formulate his own plan of action, and what follows is likely to be 100% Rider, and not something Waver cooked up. For all we know, Waver could still be sleeping.
This seems like a more likely possibility. Rider is playing dirty and doing things which he does not want to burden Waver with, but needs to be done. This is also a personal agenda for him because he wants to prove to Saber that she is wrong, and it is unlikely he has much respect or concern with the people she is affiliated with.
I also find it unlikely that Gilgamesh would go along with some contrived scheme to make Rider and Saber fight each other, when he already knows they will end up fighting anyway. It would also be unusual for there to be a sudden focus on drawing Saber out, when the immediate interest of Kirei seems to be Kariya. Why would he drop his new toy and go play with something else he has shown no interest in previously? He doesn't care about Saber, only Kiritsugu.
I also find it unlikely that Gilgamesh would go along with some contrived scheme to make Rider and Saber fight each other, when he already knows they will end up fighting anyway. It would also be unusual for there to be a sudden focus on drawing Saber out, when the immediate interest of Kirei seems to be Kariya. Why would he drop his new toy and go play with something else he has shown no interest in previously? He doesn't care about Saber, only Kiritsugu.
