Are these two sentences grammatically correct?

I still don't understand this : If Robo was alive at the end of time after defeating Lavos, why should Lucca worry about Robo ceasing to exist in a time period that comes before the end of time? (2300 AD) Obviously if Robo wasn't made in the saved future, he should cease to exist at the end of time too, after the three players defeat Lavos.

Or is it possible for someone to exist at the end of time after altering history and only cease to exist when they go back to their own timeline (a timeline that lies in between the altered year and end of time)? Do you think it'll be clear to me after I go through that article on time travel concepts?

Could someone please tell me if these two underlined sentences are correct?

For the first sentence, can I use this instead : "Obviously if Robo hadn't been made in the saved future, he should have ceased to exist at the end of time too."

PS : I know this shouldn't really confuse anyone answering the question but just in case someone is wondering about it, "saved future" means the future(2300 AD) after defeating a certain enemy in 2000 AD.