The problem with the series existing within a predestined loop is that the Xehanorts would have the entire timeline to work with. If there is no sense of "now" in the KHverse, what exactly is stopping MX from interacting with the future? He should potentially be able to travel until he stops existing. The idea that the present determines the past and future, the past is unchangeable, and the future has yet to happen creates a natural roadblock that Xehanort would not be able to cross. The present would act as a wall and provide a concrete reason why he can't see into the future. Another problem is that we clearly don't have all of the information at the moment. For example, Sora and Riku being able to time travel without giving up their bodies at the beginning of DDD, timeless river, etc.
By 3D's time, there isn't a future yet. MX literally cannot go to the future because the future hasn't happened yet. Once it has happened, him not going into the future from that point in time is already predetermined.
Plus, i guess there is a misinterpretation of time travel. You gain the ability once you give up your bodies once, which Sora and Riku did. Sora gave up his body unlocking his Heart with the Keyblade of Heart, Riku got booted into the Realm of Darkness by Ansem possessing his body.
As for Merlin, chalk it up to being Merlin. He already defies existing laws by being able to travel between Worlds without utilising Corridors of Darkness or a Keyblade to open the Lanes Between or those special Paths prepared for Sora.