How do you imagine the emotions/reaction went during/after the first showdown between the turtles and Sliver went after his escape, especially Grav? I'm personally imagining all of them kind of crashed and had really bad post-traumatic reactions, meaning they weren't really available to support each other, but thankfully April and Casey thought to check in on them and helped them all through it (or helped Grid, Shelle, and Blob enough that they could go help Grav). Anyways, you absolutely rock in general, but HUGE KUDOS to you for gracing us with your awesome Super Turtles headcanons and fic. Seriously, thank you. <3
Oof, yeah, they definitely wouldn't handle it well. It could be extra unfortunate depending on how soon they got the alert that there was trouble in the city and whether or not the alert offered enough information that they knew it was Sliver they were about to face when they were heading out to address said trouble. Even just that little bit of extra time on the way there to try and mentally/emotionally prepare themselves could make all the difference...but then of course once they get there and they're actually fighting their father for the first time in the field, trying to prepare for it may not matter.
Grid would hold it together the most and probably end up taking point during the battle since he's been ready and itching to try and make Sliver pay for what he did; at least in that moment, it'll do him some good to take out the aggression that's been building toward him.
Blob would falter, because a part of him still hasn't processed/doesn't want to believe that this is their life now, that this was their father and now they're enemies. It hasn't had enough time to sink in; he doesn't want to be fighting him.
Grav would be triggered just seeing him face to face again (much like Leo seeing the Shredder's elite guard again after they ambushed him.) Plus it could very well have been one of Grav's first times back in the field after recovering, so that would make split second fieldwork decisions a lot harder, hence Grid needing to step up.
It's hard to say how Shelle would perform; it'd probably depend on the specific parameters of the battle. Like if Sliver's creating any kind of technological or electrical crisis for him to focus on, he can compartmentalize and do what needs to be done. Ending up anywhere near water, however? A fire hydrant bursts as collateral street damage or an antigravity push from Sliver nearly sends him into a park fountain? Forget it, he's seizing up.
After the battle, you're totally right, they would all be a mess.
All of Blob's grief would flood right back up; he'd be so upset and frustrated, probably crying and kicking himself for hesitating. Grav would feel crushed for letting his fear get the better of him, especially with the added shame of being out in public where the civilians could see him at less than his best, and has probably been stuck in a panic or anxiety attack the whole way home - that or he might just start shutting down and, of course, isolating himself, because that's what's been instilled in him now when he's vulnerable. Just hide.
Shelle would waver because he wouldn't be sure which of them to prioritize to console, unless it's a case where he's been triggered too and he can't help, and Grid feels totally overwhelmed watching his brothers crumble like this because he may be good at coming out swinging in the field but he doesn't know what to say or do to make it better now, if there's anything he even can say. He may end up paralyzed by indecision or scrambling to make it stop, make it stop, and end up making the pandemonium worse and then feel horrible and guilty for it. Ultimately everybody spirals into anxiety and/or panic by extension.
I like your idea of April and Casey helping them afterward, I could definitely see that. April might handle PR, try to get any reporters off their back by making her own statement as mayor while Casey tried to do damage control and help the Supers pull themselves together behind the scenes. Reminding them this isn't their fault, helping Blob calm down, reminding Shelle he needs to take care of himself first, giving Grid tasks to do like bringing blankets, tissues, a cup of water and such. Actions speak louder than words, after all, and then maybe Grav can be lured back out. A nice pot of tea and painkillers passed around and quiet time for everybody!
And because this is early on and it's going to take some time for the turtles to build trust with them beyond a working relationship, it's probably only after they leave and it's just the four of them that any of the others aside from Blob would be willing to have a good long cry.
...You're kinda making me want to write it now XD We'll see. I'm just so glad people enjoy my writing for them as much as I do, it's so much fun exploring such an underrated part of the canon. Thank you so much for giving the Supers and my interpretation of them a chance! I love them and I really love sharing them with all of you! <33