Maybe someone here can finish and give me the final answer. Here's how far I got.
1. The progression of age difference can only be unequivocally determined at the valid end of the spacetime path so long as the start was valid as well. Validity of the start and end is co-location of the two parties. The reason it can't be determined during the path is that the age of each participant is subject to perspective when separated. All perspectives are valid so there is no universal answer on how age difference unfurls from all perspectives except during co-location of the parties.
Another thing I learned is that age difference can't be established by light signals between the two parties if the change in relative velocity is a relative stop. The reason is the separation between them can register different age difference results from other perspectives. Also, in constant relative velocity, only reciprocal time dilation is going on. Age difference is not 0, it is dependent on perspective so each of the party's reciprocal view of the others time being slowed do not cancel out to no age difference between them because other perspectives can say there is. These two facts were very important to discover.
2. I don't know where acceleration fits into this. I was told this was a non-moving, non-inertial acceleration like gravity; It is not like the changing rate of velocity over time type of acceleration. The example was people on opposite sides of the earth feel the acceleration due to gravity but are not moving wrt each other. That same feeling of non-inertial acceleration is felt by a person making a change in velocity because she is not moving through space within her own frame yet she feels the force of her acceleration which is seen by an outside observer. I'm not sure if that gravity feeling establishes she is the one moving through space and therefore permanently less through time than the other party. I'm not sure if the force that's felt causes permanent age difference as is caused if you were trapped on the surface of a Jupiter where time would continue as usual for you but someone outside who could observe you would see your time slow. If you left Jupiter, you would have aged less than the person observing you. You'd have to feel some pretty heavy gravity over a long time for that to be the cause of all the age difference, but maybe that's what's happening. Also there are examples of clock info handoffs where there is no acceleration and yet age difference occurs. I thought we were working towards some understanding until this non-inertial gravity thing came in at the last minute and I got banned before my questions about it got answered.
3. I never got definitive confirmation that all other theories of how age difference occurs or unfurls are false or not. These include adding up the age difference at the end of each leg of the journey (not valid spacetime paths), equating reciprocal time dilation with age difference when a change in relative velocity occurs (right answer for the wrong reasons), using the sweep of the changer's line of simultaneity to calculate the other's continued ageing while the changer doesn't age during the sweep (doesn't work because it determines age diff before the end of the spacetime path) and a bunch of other theories I made up where the math worked perfectly and seemed plausible but contrary to spacetime path rules. One of them was the changer became the preferred frame and the math worked very nicely for that wrong idea.