I haven't read every single reply, but what strikes me is that the response is oddly drawn out and seems very emotionally fraught.
IME if a 5yo hits, the most effective response is to immediately and sternly tell them no, separate them from the other child, give a short, mild and immediate punishment (e.g. time out/sit on stairs/send to room) and after this, prompt to apologise and inform if there is any further consequence, such as loss of a privilege. IME the short time out in the moment is usually enough - I only use loss of privileges to address issues which seem to have become a kind of automatic pattern.
If they are still within sight/earshot of you, then slightly play up the sympathy towards the other child (unless other child was winding up their sibling hoping to get them into trouble - which does happen - then they would get told off as well).
If 5yo is upset by this, that is not really a problem, in fact it is a good thing if they are upset because it shows they know that they did something wrong. However if they are escalating into hysteria and extreme distress, then it's clearly not productive and you might need a different response, and probably a more proactive management plan as well.
I would also not leave it for 30 minutes, and they don't need telling off by multiple parents. That is too long and making it into far too much of a drama. A 5 min time out is fine, maybe 10 mins max if they are angry/protesting being sent away or the other child is substantially upset and/or I need time to calm down myself. But don't just abandon the child who lost control either. Give them a short period of absence but then go and talk calmly to them. I would also not get into a stand off with a 5yo about not being able to enter a room with their sibling until they apologise, this is silly because it's impractical and you cannot force him to apologise. You can prompt him to apologise, explain why he should apologise, and you can punish him for not apologising if you feel that is right, but you are not in control of his voice box. And I do not enter stand offs with my DC over things like that, and especially not at age 5. I do appreciate though that sometimes in the moment we say something and then realise it's a silly thing to say but don't want to backtrack. But as a general rule, don't get into stand offs like this. It doesn't help. I have way more to say about the whole apology thing but it will have to be a separate reply, and maybe tomorrow because this is already really long. But in general, keep the response short, predictable and simple. Don't drag out punishments and stand offs, it gets you absolutely nowhere.
As for whether it's normal/typical behaviour for 5, I think it's completely normal for siblings to wind each other up and argue and this can get physical, obviously discouraging it is important but I do think it is completely normal for siblings to thump each other from time to time. IME though it is not normal for a 5yo to hit adults especially grandparents, sometimes they do get stuck in a habit of hitting a parent who hasn't put their foot down about hitting when the child was younger (e.g. 2/3).
But he isn't your first child - did your DD hit at 5? She has the self control at 9 not to hit her little brother, who is probably the single most annoying person on the planet to her. He's not a tiny baby either. It would be completely understandable if she hit him back and yet she doesn't. So at some point between 5 and 9 hitting goes from being "is this not typical?" to definitely not typical - when?
The other thing that makes me pause here is your description (taken from a few posts)
He's usually quite upset and scared himself after hitting anyone...He obviously feels bad but he refuses to apologise or even ask DD if she's ok...He said he can't do it...I doubt that in that moment when he's in a rage he is actually capable of stopping himself from being violent. The TV thing works as a consequence with everything else. In this case it doesn't work though because I think he's just too overcome by emotion to think rationally at that moment.
He was scared and upset. Not scared of us or his sister but maybe of himself and how to deal with his emotions and the situation. And I think he does get scared that maybe we don't love him when he lashes out. He was clearly very, very relieved when DD hugged him.
This to me sounds different. Both DS1 and 2 would be like this. I didn't understand it as much with DS1 but I recognised it more with DS2, and he gets it even more extreme than DS1 did. If he got to the point of hitting and you address it sternly or punitively in any way, it would put him into an extreme distress response and if you tried to get him to do any kind of remedial action (e.g. apologise to someone/clean up a mess made in anger) then it could start a chain reaction by the end of which he would have absolutely no clue WTF you were even on about. It was so obviously counterproductive that we had to stop and look at different approaches. Both DS1 and 2 have ADHD, DS2 is probably also autistic but not yet assessed. And no, before everyone starts on me, I am NOT saying that 5yo hitting = ADHD and I am NOT saying that big emotions over being told off = ADHD either, I am just saying that my 2x DC with ADHD would go into this kind of emotionally dysregulated state which might result in behaviours like hitting, and then would likely escalate if they got told off or punished for it. I guess I am saying that this "loss of control rage" type state is not typical, and the extreme emotional response to being told off or having lost control is also not typical. IME.
To contrast with DS3 who has just turned 5 - if he hits, he gets told no and sent to time out, you don't even have to take him, he just goes, sometimes angry and stomping, sometimes immediately upset - then he cries, he gets prompted to apologise, he apologises, he will be a bit upset in the moment, but everyone moves on, nobody feels unlovable, he is much less likely to hit for the next few days/weeks until he forgets again and over time it gets less and less frequent, I expect it will eventually stop. That's literally it!! No hours-long drama, no stand offs - he makes a mistake, feels bad about it, tries to make up for it, we all go on with life and the whole incident is over within a few minutes. It's like night and frigging day.