Why did he burn a swipe on that Doomsayer instead of just Silencing it? That play made no sense to me. Freeze mage doesn't really have anything that can buff a 0 attack minion?
Yup, by far his most crucial mistake as it resulted in him not having an answer to Alexstrasza.
I didn't watch that game but Hyped is known for having a mage deck that runs Twilight Drakes.
I could see that had Stunner not
a) thrown away his other Keeper one or two turns earlier when he bombed - not silenced - Hyped's cycling Thalnos just to prevent his Yeti from taking a single point of damage, which was extremely unlikely to matter as Flamestrike was still turns away, leaving Hyped with plenty of time to ping it down to 4...
and b) still used the Keeper to clear the Doomsayer. What he did was, instead of simply silencing the Doomsayer, he silenced his frozen Yeti and used its attack plus Swipe to kill the Doomsayer. He literally wasted a Swipe to remove a worthless 0/7 body.
Thinking about it he really was most likely expecting the Giants/Drake Mage and afraid of the Doomsayer getting taunted up later on, but even then it's pretty damn careless to burn your removal that early when you have enough power on the board to punch through the taunt anyway.