Your timeline is flawed; Wasn’t Winnie in Europe for eight years.
No. The narrative in the final moments of the final episode of the original series stated that Winnie went to Paris to study art history; that Kevin wrote her every week for eight years; and that his wife, eight month old son and him were there to meet her when she returned.
Except for the first declaration; the others are open to interpretation in the order that they occurred. Being that writing was still the cheapest form of communication; it's not unreasonable to say that Kevin would have continued to write in lieu of expensive long distance phone calls.
Further; one can get a Masters In Art History in just around two years. If I attend MIT and receive an Engineering degree; do I stay in Massachusetts? No; I move back home or I move to where the work is. Why would Winnie stay in Paris for an additional six years, just to give up and come home for no reason?
So; having had finished her pursuit of an art history degree; and for the purpose of the spin-off, additionally earning herself a doctorates in mathematics (which can be done in just five years); Winnie comes home. Kevin and his wife meet her at LAX (because Kevin and his family live in Los Angeles) to pick her up and so they can visit before she moves on. Kevin continues to write Winnie until his wife passes away and he goes into a state of depression.