>Like how could Hawkeye know spatially where the portal would
>open in relation to CAP?

I think you mean Falcon

His line (On your left) was a callback to Winder Soldier when they first met and cap kept lapping him at the Washington Monument

>Why did they give BP and crew 20 seconds on a casual stroll
>and he smile directly at camera?

C'mon mayne. S'called a "Hero moment" which allows for a dramatic stroll/entry

At that point they were highlighting "look...everyone is back" as a payoff for the anguish of having to watch them all sequentially fade away

>Why did he says "Avenegers Assemble!!!!" *after* they had
>already assembled?

"Avengers Assemble" isn't a call to gather, its a call to proceed as a group (think: "charge!", see also: "Imperius rex", "it's clobberin' time" or "hulk smash!")

>Thank goodness Thanos give them the 90 seconds to line up and
>not start attacking earlier.
>Just seems to slow down the action for pure fan service.

...or you could see it as a "breather". we had just seen cap/ironman/and thor get beat. give the audience a moment to recognize this and just when everything looked its bleakest and here comes the cavalry

that is a pretty fairly standard story trope (particularly in comics) - see:

https://allthetropes.fandom.com/wiki/Just_in_Time