past simple vs. present perfect

[P_a_v]

Hello all, I came accross this sentence, in which a past simple would be the only correct choice. However, according to the results, present perfect was used.
Can you please tell me why?

A year ago he had been regarded as the company´s rising star in the export department.

Another question is about an idiom:
"Put your mind at rest". This one is undoubtedly correct. What if I use "Put your mind in rest" though. Is it acceptable also?

Thank you !

[Mister Micawber]

A year ago he had been regarded as the company´s rising star in the export department. -- The only reason to use past perfect here is to contrast it in a greater context with another, more recent past event, as in:

A year ago he had been regarded as the company's rising star in the export department, but he fell from grace in June.

What if I use "Put your mind in rest" though. Is it acceptable also?-- No.
_________________
Native English teacher at Mister Micawber's