The young man in the dinner jacket was - according to Nico - Nico. Search for "Cardus" in the database and you'll find the following (in a letter from Nico to me, circa November 1975, i.e. very early on in our entanglement):
"There's a superb 4/5 pages of a sort of nightmare weekend at JMB's Adelphi flat [in Neville Cardus' Autobiography], which is vividly "true" and to me extremely funny. I in fact appear in the account though no one would realise this, but it brings back so very vividly some of the scruffy things I took absolutely for granted in those days. It's certainly an unknown side to JMB! Yet Cardus wrote me shortly before he died last year saying how right I was that Barrie was a man of great wit and humour."
As to the unwashed/ancient shaving brushes, they may well have been Michael's, although prior to your post I'd always assumed they were Barrie's. Of course we all know how people cling to the relics of their dead loved ones, often preserving them as they were left. My son Anno died a month short of his 21st, and for years I clung to his "last" ashtray, complete with the butt-ends he'd stubbed out before heading off to get himself killed ...