Mammillaria karwinskiana (one of the longer-spined forms)?

Yep, that should be a Ferocactus.

No; some sort of Echinopsis/Lobivia

Euphorbia trigona (so not a Cactus)

To me it looks and sounds ("appearing suddenly") like stress 'damage' (sun, temperature, ...).

If you want you can try and look for insects with a magnifying glass.

I found this one in a tray of A. xylonacantha. It's obviously something else; it grows a lot faster as well (this is it at it's third year starting). The closest I could find is Agave guiengola, but since I don't have any serious Agave-ID experience I'm asking here: http://i679.photobucket.com/album...

I think I can see some hairs on the areoles? => Pilosocereus
B. hertlingiana has fewer, but stouter spines.

Completely normal, Ferocactus seedlings are big and grow fast. One of the reasons they're excellent plants for beginning seed-growers.

Well, some sort of Mammillaria, but I'm no good with those :p

Agree, but that is a nice plant!

#1 a red flowering M. gracilis clone?

1) Mammillaria karwinskiana
9) Is it a cactus? An epiphyllic one (lepismium, ...) maybe?
10) coleocephalocereus? the hairs are too 'bristly' for an espostoa (I feel)

a very nicely spined Parodia I think; P. aureicentra?

that's very nice!

yep, Pilosocereus lanuginosus