As we see that Grand title can be given up so I got the thought that can the servant who gave up the title can reclaim it? It’s not like anything is stopping them is it? And the servant summoned are shadow for the original self so I don’t think that the original’s Grand title is lost permanently.
I think that as soon as the current Grand gave up or lose their title, a candidate step up and take the spot. So unless there’s more than 1 Grands each class, I don’t think they can reclaims their title.
Fate universe is made of handwaving.
If it suits the current plot, it will find a way to happen.
I can’t see any particular reason why the same servant couldn’t be summoned into the Grand role in case of future need. I mean, servants are just copies of heroic spirits, so nothing that happens to them affects their record in the Throne of Heroes. This particular incarnation of King Hassan sacrificed the title of Grand and the extra power of his spirit origin, but that doesn’t mean the Counter Force couldn’t summon another Grand Assassin King Hassan, later (or earlier, for that matter).
It wouldn’t be the same King Hassan as we met in the singularities, however, as he wouldn’t retain memories of his previous incarnations.
The only exception to this is Solomon, as his final act was to erase himself from the Throne of Heroes, and thus he can never be summoned again.
Though, since the Throne exists outside of time, you’d assume such an act would prevent him from having been summoned in the past, as well, but that’s just the kind of inevitable paradox you always run into with time travel. Best not to think too hard about it.
I don’t think it’ll stop world/dimension-hopping servants like Musashi and Arthur, so we just need to get Solomon from another dimension.
(Edit): Also, as much as I’d like First Hassan to take the Grand Title again, if it’s true that another servant takes the title when he gives it up, then I’m curious who the next Grand Assassin is…
Supposedly, the Throne exists outside of dimensions, too. After all, EMIYA was a heroic spirit from a different timeline in F/SN, and we can summon Musashi, who was from a culled timeline. It’s also implied that Nameless from Fate/Extra is the same EMIYA as from F/SN, even though Extra takes place in an alternate timeline in which he may not even have existed in the first place.
In short, I think it’s probably impossible for us to speculate on what is or isn’t possible within the Fate setting, as internal consistency has seemingly never prevented the writers from doing anything they want.