My version of Original Timeline (including headcanons), simplified:
1944 - Charles meet Raven. Erik's mother killed by Sebastian Shaw.
1962 - First Class.
1965 - Charles opens school. School get closed, because of Vietnam War, then Charles Xavier, fall into depression.
1966 - Beast's create serum to Charles regain his legs. The serum made him have working legs, but, he has his telepathy ability "weak". Eventually Charles start to becomes addicted to serum, which made he lost completely his powers.
1973 - Mystique killed Bolivar Trask. Logan and Sabretooth joins Team X. Logan leaves Team X. Trask learn his son Jason is a mutant.
1975 - Charles Xavier began taking healthy doses of serum, returning his telepathy ability, but ''weak'', return the effect prior his addiction.
1977 - Because of effect of serum, Charles ended up causing irreparable hair loss, leaving him bald, by a result of the time he was addicted to serum, even though he has taken it correctly after stopping being addicted. Stryker learned about a person called Charles Xavier, he then seek his help. At time, Charles didn't had reopen in school yet, he reopened in 1979. But even so, he decided to help Jason. Xavier just teaching him for a short time, because Charles didn't do what he wanted. Stryker wanted Charles to cure his son, Charles encouraged Jason to embrace his mutation.
1978 - Mystique manage to escape of Trask Industries of her own (So Mystique stayed 5 years in custody of Trask Industries). Charles finally take courage to look/see Raven, when he did, with Cerebro, he discover that she escape Trask Industries custody, then Charles and Raven sibling relationship officially ended, as he first comunicates with her, since 1962, with Cerebro, projection him to her (like he did in DOFP), they discussed, end their sibling relationship. Raven already have changed to Mystique of Original Trilogy. Jason Stryker causing his mother killed herself, because of Jason's illusions. Stryker frozen his son (After X-Men Origins he unfrozen him leading his status in X2). Erik released from prison, after get proved he is inocent of didn't kill JFK. Charles and Erik makes amends.
1979 - Xavier reopen school. Jean Grey joins school (The Last Stand ''20 years ago'' flashback). X-Men Origins main events/Scott joins school. Ororo joins school.
1980 - Charles become immune of serum, because of his period of addiction, returning to wheelchair. Xavier decide to restart X-Men again, with Scott, Jean and Storm, and Hank/Beast.
1985 - Hank leaves X-Men to focus on mutant politics, but still maintain contact with X-Men.
1988 - Logan get captured by Stryker, get a second coating of Adamantium, that covered adamantium bullet in his head (that's why adamantium bullets didn't appear in X1 in X-Ray). It's Weapon X's X1/X2 flashbacks (line up with ''15 years line'' in X1/X2).
1990s - Victor Creed used a drug to enhance his mutantion, leading him to become Sabretooth of X1.
Late 90s - Charles ask Hank, to help him and Erik, to built the Cerebro of original trilogy. Charles and Erik went to a big disagreement, made Erik leaves X-Mansion.
Early 2000s - Erik returns to be Magneto, Erik recounters Mystique, he restart Brotherhood of Mutants, with Mystique, Toad and Sabretooth. Erik acting behind the scenes, while Brotherhood doing field work.
2003-2006 - Original Trilogy.
2013 - The Wolverine.
2023 - DOFP's dark future.