[Disclaimer] I do not own the new Mage.
I would say that the Attainment would only prevent the tampering if the change made to the past/future timeline would itself alter Fate. It's also important to note that not everything in a life is Fate. No one is Fated to drink orange juice for breakfast the morning of June 2nd, 2021. If they are, that orange juice will be a life changing experience and not just another tasty breakfast beverage.
If a Mage is Fated to kill his father (a noted Nephandus), then attempts to travel to the past to kill the man would be thwarted. Likewise if the Mage was Fated to save a life or achieve enlightenment at a location- attempts to kill that person or ensure the location was never built would fail.
However, changes that do not directly impact Fate are fair game. The Mage's mother could be killed in the past, for instance, or the person could be manipulated to not WANT to be saved by the Mage.
I would say that suitably indirect measures could be employed via Time Manipulation as well. If the timeline were altered so that the Nephandus showed up with 10 backup goons, the fight would be harder, yes, but it would not necessarily be "tampering" with Fate.
This would also allow Fate / Destiny to 'spring back' in unexpected and fun ways. If the Nephandus gets goons, then the Mage ends up with assistants of his own- possibly making the battle far more epic and even easier for the Mage than it would have been before the tampering!