Hi,
Welcome to the Forum.
1- the money he hadn't spent he was allowed to keep. It's better to say he was allowed to keep the money (that) he hadn't spent. Restrictive
2- the fact that he was only sixteen didn't really matter I don't think we normally think of this 'the fact that. . .' structure as introducing a restrictive clause, but it does seem to be restrictive.
3- the boss, who had arrivd at eight in the morning, was still at her desk at midnight. is this non-restricitve? No.
also when is says give the passive counterpart..what does it mean.
for example:
the emperor presented the visitor with a gold medal. is this even a passive sentence? No. Passive is, eg A gold medal was presented to the visitor by the emperor.
Best wishes, Clive