It's established that Griffin is able to turn cloth invisible. One of his earliest tests is on a piece of linen. So why doesn't he make invisible clothes instead of running around naked?
It's because Griffin doesn't get a chance to plan that far ahead. He's about to be evicted from his flat, and ends up barely having time to change himself, let alone clothing and supplies. After that, his paranoia takes hold and he burns the whole place down, so he loses his equipment. Once he finally gets to Iping, he concentrates on a cure, rather than reproducing the results. And considering how the mechanisms necessary to render objects invisible were obtrusive enough to bother a whole building, there's no way he could've done that without getting Mrs. Hall involved.
Also, how would he then be able to find his invisible clothing?
Does Adye die or not? He's described as being shot by Griffin, and falls down and doesn't get up. When asked, Kemp says, "He's killed Adye, shot him anyhow." Kemp consequently seems uncertain if he's actually dead. In the epilogue, it's mentioned Adye questioned Marvel about Griffin's notebooks, but this could've happened prior to him getting shot (i.e. perhaps Kemp told Adye about the books, and that Griffin said Marvel had them, and Adye asked Marvel, who denied having them). The fate of this fairly important supporting character being so vague always struck me as a bit annoying.
The Quicksilver residue. Any amount of liquid that could cover a grown man's body is bound to leave large piles of dust. And, as the Keeper says, the residue takes 2-3 days to biodegrade. Wouldn't it be a security risk to leave all this Quicksilver just laying around?
Sure, but what choice do they have? Darien's usefulness outweighs the risk.