The [5] is from Ketsui. Unless the artist says it's from something else, I'm adding the Ketsui tag.

I don't recognize the outfits as one from any characters in a CAVE game(that I know of, anyway). Are they alternate costumes or are cosplaying?

Mystia's cosplaying princess Reco, from Mushihimesama (the skirt and the design of Reco's upper clothes were the clue, despite the different color on the skirt), and Wriggle's cosplaying Seseri, from Espgaluda (the clue is her wings, and then, the clothes)

okay, looking at the pixiv tags on the picture source, the artist has referenced Espgaluda, Mushihimesama, Battle Garegga (I thought that the ship behind Wriggle was very familiar.. turns out it's indeed one of the Black Heart models -- the first being the stage 5 boss, and the second being the midboss before the final boss, Glow Squid) and Cotton (I'm clueless about how it's referenced here)

I think that the pink item near Ketsui's point item is indeed a Perfect Cherry Blossom's cherry point item..

The tea cup references Cotton. (TEA TIME!)