With regard to the 'I don't necessarily know or trust you' 'so don't send your kid to sleep at my house then' thing:
I have 3dc who all go for regular sleepovers, or host them themselves.
I don't know all the parents at my dc's sleepovers because why would I? They're probably perfectly nice, but I'd be a fool not to acknowledge that they might not be. Equally, not every kid invited to crash at mine has parents who have had a long acquaintance with me. Why should they generously assume that I'm definitely risk free? I might quite easily not be, or have a dp who isn't.
The OP has already made it clear that she doesn't know the parents of the new sleepover kids. So, they don't know her or who is in her household, either.
If you have a house rule that 'there are no sleepovers unless I personally know the parents' then that's entirely reasonable, I guess, but I'm not convinced it's much of a safeguard.
You have NO idea if the host's dad has a tendency to grab his dd's friends' bums if he passes them on the landing on their way back from the loo, or if the host's older brother might be joining your ds & his mate & encouraging them to watch a film you wouldn't sanction.
You are in the position of balancing risk (bad stuff might happen, but I've mitigated it because I know the parents & they seem nice) against restricting your dc's social life (bad things might happen & I'm not prepared to risk that). There aren't easy answers because you CANNOT say that the adults in the house aren't a risk. Simple as that.
The phones thing is almost a side issue; yeah, un policed phone use might open them up to well, exactly the same risks that you accepted when you put that phone in their hand, except it might happen after midnight 🤷🏻♀️
As I've said before, I think 'no phone use in the bedroom, you'll be up all night & might egg each other on to risky unreasonable behaviour' is quite sensible.
Confiscating phones is too far. On a practical level, it's also completely unenforceable for guests 'oh my phone? I left it at home actually.' Or 'oh sure, here you go!' .
I'm all for pragmatism with teenagers...pick your battles.