First, there are NO isntant casts, the quickest you get is 0.1 sec castime. So it would be possible to time it good yeah.
Even then
As StAcK3D_ActR says. Deep Freeze works in a quite weird way that takes getting used to.
For example lets say it increases cooldown by 150%, and lets say sednas heal is 10 sec cooldown.
0 Seconds; Sedna heals herself
10 seconds; sednas heal CD is finished,
12 seconds; Sedna Wanna cast her heal, She is hit by deep freeze, which changes the cooldown of heal to 15 secs,Sedna now got 3 seconds left on the heal Cooldown.
15 seconds; Sednas heal cooldown finishes
16 seconds; Sedna heals herself
It gets even more absurb once you take the deep freeze duration (think its 8 seconds)into account.
0 seconds; Sedna heals herself
1 seconds; Sedna is hit by deep freeze, which changes the cooldown of heal to 15 secs.
9 seconds; Deep Freeze wears off, sednas heal cooldown is reset to 10 seconds
10 seconds; Sedna heal cooldown finishes
11 seconds; Sedna heals herself 
What deep freeze really does can more accurately be described as "disabling the target from using an ability he used again for 8 seconds"