I've been an orchestral musician for nearly forty years and in that time we've never done a concerto in performance without a rehearsal beforehand. I find it hard to believe that this video represents a performance - Grishka's comment that it was at a rehearsal sounds right to me (and rehearsals are sometimes videod for documentary purposes.)

As for memorising a concerto, I'm not a soloist and I could probably play at least a dozen concerti from memory and so could most of the musicians I know - it's no more a feat than a folk singer getting up and doing a set without a crib sheet.

And viola players - one of the older Groves editions poses the suggestion that viola players are simply horn players who have lost their teeth and need to find something to do to earn a crust.

Cheers,

Weasel