But I don’t buy the half-reassuring line that says X has set his sights on Europe simply to compensate for his supposed estrangement from the Orange Man.
Your interpretation is basically right. X is acting, not acts, though.
She does not believe the interpretation made by others that X decided to focus on Europe only for the purpose of compensating for (making up for) for fact that he is supposedly estranged from Trump.
to buy a line= accept an argument or interpretation
half-reassuring=that makes something not "all bad", an x that is partially re-assuring
set one's sights on something=is taken from sights on guns or other weapons or telescopes, to have something as a target
Sorry, but I do not want to use those names as I do not want to be associated with those characters, even anonymously. :)
Something someone says or does can be entirely or partially re-assuring. half-reassuring means partially. It is not fully reassuring.
In other words, if X was only spreading his ideological crap in Europe due to his estrangement from the Orange Man, it would be half-reassuring in the sense that it would not be only for other reasons. She thinks it is for entirely other reasons. He is on an ideological mission. [what a hateful character he is].