Anime adaptation -lancerman wrote: -Gero who admitted to not watching anything after Namek decided to alter his attack to be 19 and 20 instead of 17 and 18.
-17 and 18 are far less evil than their counterparts
-A disease Goku had occurs later
Gero didn't decide anything different. In both timelines, it was Gero and #19 that first attacked the island city. F.Trunks was simply mistaken (he didn't know that a #19 existed, or that Gero had been converted). The scenarios played out in a similar way in both timelines, #17 & #18 were in "sleep mode" and were last resorts because of their disobedience (both timelines). Gero, and presumably #19, suffered the same fate in both timelines.
#17 and #18 are the same, but they developed different characteristics in their respective timelines because Goku had survived in the main timeline, and Cell had arrived. This led to #17 and #18 (main timeline) with #16 having their priority be to have fun in tracking down and eliminating the alive Goku. Their future counterparts had no Goku to put down (and no Cell as a threat), so after they killed off the remaining warriors, they were probably bored and decided to go recklessly "play" with the rest of the world over several years, giving them evil characteristics.
I don't remember Goku's disease being explained, but to head-canon, it was possibly seeded during the events of Namek with the emotional beating his heart took(?), which possibly made it vulnerable. Beyond that, he had been away from Earth in outer space for over a year (space virus?). The disease taking hold of Goku at different times is a result of F.Trunks warning, which created an alternate path. In the space of those three years of preparation, Goku had trained and strengthened his body/spirit - in base - to higher levels, which possibly enabled him to withstand the effects of the virus for longer than he should have(?).
I don't think that Toriyama was ever trying to send his audience into a head-spin. Space travel is probably something that's easier for us to work out, but I've never seen anyone have issue with how everyone found their way to Namek, and under varying times. We just said, "ok".