Pursuant to your last comments, you argue that both statements are true, thereby proving, if you will, that there is a single reality. While it IS true that Gaeliss exists from the point of her actual birth (c. 1940) to 1968, and it IS also true that she existed from 1730-1745, what CANNOT be true is that this occurred within one single reality. One can only "see" these two facts as occurring along two separate timelines - two histories - since before Gaellis went into the past, her history was different. She did not exist in 1730 - she ONLY existed in the 20th century. It is only AFTER she TTs to 1730, that she creates a new timeline for herself. We have a similar case with Claire. We know that she exists in 1945 in Scotland, and we know she exists in 1745 Scotland. However, at the moment that she steps through the stones, she literally, not figuratively, ceases to exist. This is not to say that she was not ever really in the present - to be sure, there are birth and marriage records...Frank, etc. But she is gone, nonetheless, from her original timeline. This is also true of Gaeliss.
You ask me at which point Gaeliss would have had to go back in time in order for her to precede Roger and his line. I answered too hastily perhaps, claiming that it would have to have been prior to Roger's birth. My omission of the rest of his family line was for simplicity sake, as I assumed that it would be obvious that Gaeliss would have to have TTed back to the moment - of necessity - before she gives birth to her child with Dougal, which becomes Roger's ancestral line. You are right to have pushed me on that point. That said, in doing so, it exposed an even greater error than I had first imagined. Gaeliss must have existed before having given birth to Roger's earliest ancestor - save herself and Dougal. Yet, until the moment that she TTs (1968), she ONLY exists in the 1940-1968 timeline. However, Roger also exists in 1968, along with his entire family line dating back to 1745. Therein lies the contradiction - that Gaeliss cannot pre-exist herself in her own 1940-1968 timeline. To do so, is a metaphysical impossibility.
I give you this example for your consideration: instead of 1968, let us imagine that Roger and Gaeliss meet up in 1967. They strike up a conversation, and Roger tells Gaelliss that she looks identical to his GG 7x removed. EXACTLY the same...like an identical twin. She replies by saying that she has never been outside the walls of her city. Yet, Roger exists in 1967, which is impossible given that Gaellis did not yet exist in 1750, but rather, only existed in her own 1940-1968 timeline. You may then ask again, "At what point would Gaellis have had to TTed in order to resolve such a dilemma?" The answer is simple - there is NO POINT in Gaellis's original timeline for her to go back in time in order to resolve this contradiction. Thus, while there is no evidence of an any secondary timelines/history (or an alternate universe) as presented by the author, the resulting inherent contradiction as presented above does not allow for any other interpretation.