Regarding Time Travel
In the Solas Tempus universe time travel is a regular plot element, this makes it important to have some kind of standard method for dealing with the mechanics of time travel.
Time travel is performed through various methods, discussed elsewhere. It is, however the doorway to issues of altering timeliness and not only how a change in the timeline happens. Important to also have at least a word or two about fate and time.
As it pertains to time travel, fate governs as an unseen force that needs to be poorly understood, but is also flexible. For the purposes of game play and story plot, if you return a series of events to about the proper sequence and circumstance of events fate will adjust the major events to fit in the way they are supposed to. Secondary and tertiary people involved, exact dates and times of contributing events, and other specifics can alter and shift around central events - these are flexible bits.
When something in the timeline changes the alterations do not happen immediately but occur like a wave on a pond, traveling forward through the timeline. Ripples move both through three dimentions outward from the location of the change and through time simultaneously.
The kinds of alterations that the antagonist makes should also be subtle, game-changing events like the Nazis win WWII or Russia attacks during the missile crisis are too big, too vast for most storylines, too many moving parts and too many variables. It makes for an easier story to make the changes subtle and smaller, then individual changes can be grouped in a large arch over several stories.
For plot purposes the heroes need protection from changes becoming free to act on the changes. Of course, there should be some urgency to fixing things, something to force quick action. Quick action supplies drama and intensity to the story, gives the heroes a chance to not know everything and be unprepared.
For this reason shielding from changes in the timeline causes its own ripple effect moving against the ripple of the changes. Depending on the quality of the protection, the ripples of change move around the shielded object delaying changes around the protection. Example... A ship is protected, as the changes ripple around the shield a bubble around the ship starts proportional to the strength and quality of the shield that moves inward. The alterations in the timeline don't stop fighting against the shield.
The more incongruous the changes from the normal timeline, the harder the the changes are to protect against.
Devil in the Details
It is important not to focus on certain details. This includes flexible bits, of course if something is altered once it cannot be put back perfectly. Logistically once the timeline is fixed, the smaller alterations (AKA the flexible bits) are allowed to flow and set, they are not shielded against.
Those involved in the fixing of the timeline also can optionally be allowed to remember some mix of both timeliness or just one. This allows some plot for fixing some big problem but then having to fix a flexible detail that changed the course of a loved ones life.
The exact how of the technology should be seldom discussed, the central theme should be the story line.