Quote from Anara_of_the_Old_Ways »
How Ugin's story handled time travel is with much less care. As with that time travel magic that Ugin has, why not just send the Gatewatch back in time and destroy the Eldrazi Titans in the same method done in the present timeline? This reverses the damage done by the roils, the eldrazi, Nahiri being locked in the helavault for 5000 years, that she wouldn't have summoned Emrakul on Innistrad and a baker's dozen at minimum of other plot points. This also means that if the timeline were to repair itself, the gatewatch would cease to exist by that extension and be reborn in timelines that didn't incorporate them traveling to Zendikar. Chandra would still be at the Monastery, Nissa still on Zendikar, etc. That other people would have done what they did. Time travel when done in the way that Ugin did creates plotholes in the past, present and possible futures for other storylines without even intending to.
I can't really speak to the rest, but Ugin wouldn't send the Gatewatch back even if he could. He never wanted the Eldrazi destroyed in the first place -because he's a "Big Picture" kind of guy, and didn't know what purpose (if any) the Eldrazi served, or if there might be consequences in killing them- he only wanted them contained.