About Moria:
In theory, couldn't he one-hit kill anybody right here by severing their shadow? The sun is out. Of course, I don't think it's plausible that Oda would do that to any significant character.
About getting to the New World:
The Strawhats experience at the archipelago was definitely abnormal. The only reason why an admiral was dispatched to Shabondy was because he was summoned by a world noble. Sentomaru and the Kuma clones were part of his landing force and are not a regular presence. I would expect marine interference to be around or a bit stronger than what we saw at the auction house if the government was not busy dealing with Whitebeard.
As Carnage and Assertn have stated, Rayleigh is most definitely not the only coating mechanic on the archipelago.
About the strength of New World pirates:
I think it's safe to assume that the other three emperors have crews and allies comparable to but weaker than Whitebeard's army. However, it would be a huge leap of faith to think that most New World pirate crews can match an admiral's strength. The only people we've seen an admiral fail to one-shot are Rayleigh, Whitebeard, and Marco.