The Before And After Paradox

I am trying to clarify what I now call the "before and after" paradox. This involves repeating some argumentation from The Logic Of Time Travel: Part I. However, I do not expect everyone either to read that article in its entirety or to remember all of its details. In any case, we might advance the argument. In the "Before" section, I will repeat the reasoning of Then And Now but will try to be more precise in the use of tenses.

Before
A time traveler who contemplates traveling into the past to prevent Hitler's birth, should (?) reason as follows:

either extra-temporal intervention can generate a divergent timeline or it cannot;

if it cannot, then there is just this one timeline;

in this one timeline, if anyone, whether or not they were a time traveler, had prevented Hitler's birth, then I would not exist now;

but I do exist now;

therefore, either no one tried to prevent Hitler's birth or someone tried and failed;

if extra-temporal intervention can generate a divergent timeline, then I might initiate a timeline in which Hitler was not born but that will leave this current timeline unchanged;

therefore, either way, it remains the case that Hitler was born in this timeline.

After
A time traveler arrives in 1888, intending to prevent Hitler's birth;

he remembers living in the late twentieth and early twenty-first centuries and departing from 2018;

he prevents Hitler's birth;

he believes that there is only one timeline;

he concludes, counter-intuitively, that his memories of living in the twentieth and twenty-first centuries are false.

Before And After
Before departing into the past, a time traveler knows that he remembers real events but, as soon as he arrives in the past, he does not know that. This is what the Time Patrollers say but can it be valid?