There are two types of mods. Those that run inside the Combat sandbox - they can act but they are blind. They can’t see the time or procs or buffs or cooldowns or if a spell was cast or not. GSE is in this space.
The other type of mods are outside the sandbox. They can see everything and they can tell you everything. They just can’t act on anything. WeakAuras is one of these mods.
WeakAuras can tell you something but it can’t do something. GSE can do something but it can’t be told what to doz. once combat starts it is purely blind.
As to the other part. It sounds like You have a ton of confusion about how keypress, keyrelease, pre and PostMacro work. I broke this down just recently here: Improve addon gse - #7 by TimothyLuke
There is a stack and you need to take into account both how the stacks works and how GCD abilities lock the stack.
Keypress is items that are prepended to your sequence line and KeyRelease are postpended. There can still be only 1 GCD ability in that stack. If you have GCd abilities in KeyRelease they are never going to be executed if there is a GCD ability in the stack before hand. A GCD ability on cool down blocks the stack for everything below it.