Guardian Dreadscythe doesn't special summon itself from the hand when Guardian Eatos is destroyed and sent to the GY.(it should)
Celestial Sword - Eatos doesn't activate its effect when sent to the GY. (ATK boost for Guardian Eatos).
[:tongue:]

Celestial Sword Eatos has "When...You Can" wording, if its sent by eatos effect it will miss timing because the last thing to happen is not it being sent to grave. Yes, it is an Eatos support card that does not work with Eatos effect.

Dreadsycthe though, cant miss timing so it should be able to ss itself when eatos is destroyed, though you might want to double check if there's any effect that prevent it like Utopia the Lightning etc. If not, post a replay.