Time is a matter of perspective as it is inherently infinite. It's quite possible that Alvin and GM are not the same person, persay, but rather a clone based on time, as opposed to mutations or genetics.Let's say you were to go back in time, whether by choice or not, say 40 years (assuming you are 40 years.) Now, provided you don't interfere with your family line at all, whether talking to them or not, this is going to result in "you" being born again at the exact same time under the exact same conditions. Because as far as time is concerned, "you" weren't born yet in the perspective of everyone else living in your new time period.So Jacques/Alvin is scared by some incident and goes back in time. At this time possibility, Geralt may not have come to the city, the Order didn't exist in it's present sense, etc. Very good chance his parents were killed by nonhumans, and the nonhumans may have plagued the area and there was no group to stop them effectively.Now as far as the perspective of people at this time point, everything continues as is. However, Alvin sees things differently in his time period. He grows up and eventually builds the Order the way it is in game. In this time perspective, which Geralt is in, things go as the game plays. However, naturally "Alvin" is born again because since the original Alvin did not interfere with the events that led to him being born, he was born.It's highly conceivable that this Alvin is whom Geralt is dealing with, a completely separate Alvin from Jacques altogether, and yet they are both Alvin as far as time is concerned. And that second Alvin does travel back in time, perhaps to follow a completely different path but it's irrelevant to Geralt as he continues down the time perspective he has. This doesn't discount that the Alvin that travels back in time the 2nd time does not travel into the future however. Perhaps this time as a witcher, but that's up to the imagination.The thing with Jacques trying to catch himself in later acts is that it wouldn't matter at all whether Alvin was killed as it doesn't pertain to his adult self. At worst, the new Alvin could travel back in time and kill the old Alvin, but that's highly improbable. However, he was a powerful source as Jacques knows. He could make a powerful mutant, or even a son. And if source children are as rare as they are (in fact there probably may only have been Alvin, as Ciri was gone from the world), then it doesn't seem far-fetched that Jacques had a Deterium Necklace as the Lodge probably attempted to capture him.