DS did create paradox problems with travelling through, and altering, time, but here's my take on the whole thing regarding Angelique.
Original timeline: 1795/96, curses Barnabas, is killed by him, but because she is a powerful witch she does have a semi-immortality and is quickly resurrected; she leaves Collinsport for other parts of the world, creating Endora-ish mischief here and there, but sneaks back to Collinsport every once in awhile to check on her previous "work" and, finding Barnabas still in his coffin, she goes back to harassing Samantha and Durwood; in 1968 during her latest sneak-back, she discovers that Barnabas has been released and is cured so, to restore her curse, she disguises herself as Sarina, I mean Cassandra. When all that fails, she temporarily is punished by Diabolos and then allowed to go on her way but she decides to keep a wide berth away from the Collinses.
First changed timeline: see above until you get to 1897; Barnabas has gone back in time and Quentin, along with his shifty lawyer, cast that spell to bring forth someone to help in their battle against Barnabas and guess who they get? Why her? She probably wasn't planning to do one of sneak returns that particular year; she was probably somewhere else with Hexaba but the strength of the spell grabbed her, maybe because of her relationship with Barnabas. After everything is resolved, she goes on her way. But does she still can't stay away forever so, seventy years later, she still pulls her Cassandra schtick. But then, seeing how everything she did because of her obsession with Barnabas always backfires, she decides, this time for sure, to stay away and get herself a new lifestyle, so she decides to play mortal and ends up dating and marrying a total bore named Skye.
Second changed timeline: everything from 1796 up to 1840 - she sneaks back to check on Barnabas only to discover, because he's gone back in time again, that he ain't in the coffin. When, in 1841, she's turned totally mortal, she, well, we all know what happened to her. Back in 1971, she no longer exists. Or does she?
Gerard