#1 – Where an Audio Story references a TV Adventure, locate the former as close as possible to the latter.
#2 – If Story A is released before Story B, then A happens before B, unless something in-story says otherwise.
#3 – If a story has the Doctor travelling alone, locate it in the most recently identified Story Gap.
THE FATALITY INDEX
It would seem that the Doctor’s summons to the planet of Missy’s executioners is the reason why he finally leaves River. Given this is where, and why, Nardole joins him it has to go before his first appearance as a companion in THE RETURN OF DOCTOR MYSTERIO.
FOR TONIGHT WE MIGHT DIE
I’ve decided Twelve’s cameo appearance in CLASS happens before THE RETURN OF DOCTOR MYSTERIO.