This is a very old thread, but I just saw it.
I am the technical manager of London Short Film Festival, and the author of the "how to make a DCP" guide the OP referred to.
First off, I'm really sorry to hear that the film did not screen as intended.
And why indeed did the DCP-making guide I wrote suggest adjusting peak audio levels to -10dB in DCP-o-matic?...
Is this requirement as ridiculous as it seems to be to me?
Yes. No serious theater mix delivery specifications includes such limit. Nearly all theater mix will have sample peaks above -10 dB FS.
Secondly, is this kind of level change actually normal in DCP
No. The mix should be delivered in conformance with whatever specifications apply. The ...