How Many Failed Attemps to Change the Timeline Happened?

Thanks. I don't know why I didn't think to check the DB Wiki, but after doing so I discovered there is actually a 5th one that almost nothing is known about except that Trunks traveled to that timeline and defeated Frieza and Cold. Then of course there are 2 more created during Super.

As for Future Trunks & Cell traveling back just creating random unexplainable changes that somehow made 17 & 18 stronger, I actually think it's likely that Dr. Gero discovered that Cell had traveled from the future and to assist in Cell's Perfection, he decided to make sure 17 & 18 were as strong as he could possibly make them.

Well I think the potential for their evil existed in the main timeline, but they never got the chance to be as evil as they were in Trunk's timeline since our heroes trained prior to their arrival. Also Goku being alive and 16 being awakened changed their goals. I think with our heroes being strong enough to give them a challenge that they didn't straight off kill them right away. Remember even in Trunks' future timeline, they acted like children who were playing a game. I think in Trunks' timeline since our heroes hadn't trained like in our timeline, they were too weak and weren't worth playing with, so they killed them. Then in our timeline the presence of Cell forced our heroes to get much stronger, much faster and the Androids ended up being surpassed in strength, so they weren't able to express their evil. This along with the fact that our heroes ended up showing mercy to them after Perfect Cell was dealt with allowed them to become better people

Y'know, I've wondered about this in the past and the thought just came to me: maybe it was Android 16. I think he was a huge stabilizing factor on them. That, combined with Goku still being alive so they still had a purpose, might've made the difference.

I actually think it's the butterfly effect created a series of different circumstances. The heroes were stronger in this timeline, which meant that the Androids saw them worthy as toys they could continue to play with. This allowed the heroes to survive longer and Cell's presence forced them to get stronger much faster than they did in the 3.5 years of training after Trunks' warning. This made it to where the heroes were stronger than the Androids. Also Cell being there and absorbing them meant they didn't have the chance to toy around with the heroes until they got bored and decided to kill them. Then when Cell was gone, they were already eclipsed by our heroes, so the smart thing for them was to temper they're childish destruction game desires and grow up into better people.

All-in-all it comes down to a different series of events making it not possible for them to go on their destructive killing sprees. The innate desire to survive that all living being have that may or may not have been enhanced by the cybernetic upgrades Gero built into them, forced them to become tamer beings and to grow up even. When one matures, they see that their childish desires are not good and learn to be more responsible adults. I think this is what ultimately happened. In the future timeline, there was no reason for them to mature and grow up since they were the most powerful beings around and nobody could stop them, not even Gohan and Trunks.

Interesting theory. It could work. Even without remembering who they once were, it's possible such a trauma could have had a psychological impact that would've stayed even without them remembering the exact nature of their trauma.