We're talking about FC-Origins-OT-TW-begin of DOFP as old timeline, and killing someone in 1973 what lead to release Sentinels in 2020, not about killing Xavier.
If Mysqtique didn't kill someone in old timeline and this lead to release Sentinels, then in new timeline when she's stopped before killing, it would still lead to release Sentinels. IMO she killed someone in old timeline, and we know Nixon didn't die in 1973.
If it was only assasination in old timeline, not killing someone, what lead to release Sentinels, then it's nothing gonna change in new timeline, they have to stop Trask Industries.
If Mysqtique didn't kill someone in old timeline and this lead to release Sentinels, then in new timeline when she's stopped before killing, it would still lead to release Sentinels. IMO she killed someone in old timeline, and we know Nixon didn't die in 1973.
If it was only assasination in old timeline, not killing someone, what lead to release Sentinels, then it's nothing gonna change in new timeline, they have to stop Trask Industries.