"Everyone is splendid, aren't they?"
みなさんりっぱですね / みんなりっぱですね sounds most natural to me.
If I wanted to use a particle here, I'd use が: みんながりっぱですね , because は is really a contextual particle and using it would imply a previous context instead of a simple statement, and/or a contrast ("everybody is great, unlike...").
Without context, we don't really know, but が would be much better for a simple matter-of-fact standalone sentence.
みなさん is the more formal version; it's actually sonkeigo (honorific speech) while みんな is the normal version.
^link to honorific speech explanation
Particles & whatnot aside, can anyone tell me where do I friggin use this? Humans are horrible creatures. (jk)
On a more serious note, I'd rather say "Everyone's so cool/chill/awesome/amazing", etc. And no, these can't be interchanged with splendid, since rippa literally stands to describe elegance, credibility, greatness, etc. In which case I'd say "Everyone looks so good/are dressed so well." This isn't even used in a formal setup, I feel.
**or, "Everyone's so distinguished/outstanding" (which I feel I'd mostly address to servicemen).
Yes, because the word "everyone", despite talking about multiple people, is treated as a singular for grammar, e.g. "Is everyone here?"/"Not everyone is happy." But the pronoun "they" is always treated as a plural, even when talking about one person, "The person who was sitting here left their phone behind, I hope they are coming back for it."
Except "they" isn't really a pronoun for "everyone". In fact there's no pronoun for "everyone" because it already IS a pronoun - but it's also true that it does sound awkward to keep using the word 'everyone': "Everyone arrived early. Everyone was splendid, and everyone's outfits were the height of fashion" etc etc. It's really a bit of a flaw in English that we have various workarounds for, but personally the sentence given here sounds odd to me, even though I'm perfectly fine with using "their" as a possessive pronoun for "everyone".
If I really felt it necessary to capture the ですね here, I'd probably say "...don't you agree"?