Arthur has. not been handling Morgana's arrival as well as he'd like? He'd prepared and imagined for times when Morgana could show up in Kiseki - but he always just kind of assumed it would be a later canon Morgana, aka one who hated him and remembered everything. Or from post-canon in general. So having a Morgana from so long ago for him is a little overwhelming, tbh.
He hates lying to her but he also doesn't want to overwhelm her all at once. And what do you say to someone and try to explain to them that in the future ALL THIS HORRIBLE SHIT HAPPENS and she becomes "evil" and all that... how do you say that without destroying her? How do you describe your role in it without making her hate you all over again? He's terrified of telling her they're related because he doesn't want to start her down the path somehow of like everything happening all over again - he doesn't want to go through that, he doesn't want to put Morgana through that, and he doesn't want to have Merlin have to react to all of that, either.
He knows he has to tell her the truth... but he doesn't know how much and how soon he should tell her. Until then, he's also partly grateful for this chance. That maybe he can do right by her this time around and protect everyone else better - including Morgana herself. But he's terrified, too, of messing it up.
As for others he wishes would come here -- GUINEVERE, obviously. They both were able to say their goodbyes to one another last year during fourth wall, but he still thinks about her every day and misses her. Part of him knows it's selfish to want her here to spend time with him - because he's dead back home and while he knows she misses him, he also knows that it'll be better for her, ultimately, if she can move on and find happiness and love elsewhere. And he'd never want to subject her to some of the magic of this land, either.
As for those he DOESN'T want here... his parents. For varying reasons. His father because of the distress he would cause so many people and just how utterly he would DESPISE Kiseki and everything it stands for. He knows what it would do to Merlin. To Morgana. To anyone he has CR with. If not because of his father himself, then for the influence his father would have over HIM. And, in the end, he doesn't want it to be a situation like with his father's ghost - where more and more, his father just comes to despise him.
And with his mother, he wouldn't want Ygraine to come here because he's terrified that she wouldn't recognize him or know him. Or she would, and she'd dislike the person that he becomes and is. He's terrified of disappointing her. He's terrified of her presence upsetting Aramis (since she looks like Isabelle). And he'd never forgive this world for doing anything to her, period.
Has Sherlock ever encountered some 4th wall breaking in terms of books or any adaptations of the original work? If so, what was his reaction and/or did/would he actively seek it out?