There are seven frames from when you let up on the shield button and the shield goes down. There are zero frames from when you press the jump button and you jump. Shields have already been greatly nerfed in brawl+ thanks to shieldstun, so this change is really unnecessary.
The first one you listed is indeed necessary. Having that 1 frame window where you can't input any actions... is incredibly stupid.
Is it really only 1 frame? It feels like a lot more...
Shield stun doesn't change anything about what you can do from a shield. The options OOS remain the same as in vanilla brawl just now you have to wait a bit longer to be able to execute those options.
If I'm Fox for instance and I want to shield a laggy attack I could just usmash immediately and there wouldn't be a reason for me to jump out w/ a dair. Now with shieldstun I just have to wait for the shield stun but I can still get the let go and usmash at the same rate as before and it would still be the better option at the time instead of the jump out dair.
In melee if I shielded an attack as fox I would immediatly shorthop out with a drill shine or something else, whereas if I Powershielded the attack instead I would just upsmash as I didn't have the shield let go lag. So powershielding gained a motivation behind it to learn and master for less lag that way I could do a standing attack instead of an airial.
Well in the end as long as there as some that want the code and it's not to complex, and shows some competative value, why not make it? I'm not saying it must be used by everyone or anything...
Edit: Yeah I actually usually jc usmash from shield in melee. I normally powershield dsmash with peach though which is nice to be able to do. In brawl there is no need to do that though as I can just let go.