In principle International Law says that all heads of state get all the same affordances as Diplomats, which certainly makes sense if you imagine the Diplomats as just messengers - if Bill Smith can be here because Bill is King Steve's messenger, obviously King Steve himself could come instead and isn't subject to your normal rules, makes sense.

In practice however... power matters. Eswatini isn't going to pretend it has the power to arrest Putin, but I can certainly imagine if Russia wanted to arrest the King of Eswatini they'd just do it - what's Eswatini going to do about it? If they've got a halfway plausible rationale, nobody wants to start a war over that, at least nobody who might win.

Likewise under the same legal theory neither Ireland nor South Africa could currently arrest Benjamin Netanyahu, despite the warrant for his arrest on a charge of crimes against humanity. But in practice I'm sure Natanyahu would rather not find out the hard way whether that theory holds up, in either Dublin or Cape Town. Israel has a substantial military force, and the Americans might back them, but, neither Ireland nor South Africa are defenceless and this sounds like a bad way to find out for sure.