One of the responses of Plus-que-parfait

Question

In the 2nd quiz on plus-que-parfait (La soiree de mes Reves), the 4th blank from the END requires us to conjugate faciliter in the plus-que-parfait tense.

According to me, it should be : ce qui nous avions facilite (with an accent on the last e of facilite); however the solutions key is showing it to be nous avait facilite (with an accent on the last e of facilite).

Can you please explain why we are using the 3rd person singular form of conjugation of the auxiliary verb in imparfait (il /elle / on avait) rather than that with nous (avions) here ?

Accepted Answer

ExcelinFrench 
 “ Ce qui “, 3rd person singular, is the subject of faciliter in the sentence ‘De plus, ils avaient pris en charge le buffet, ce qui nous avait facilité la vie. ‘ 
Nous is an indirect object pronoun here as the direct object of the verb ‘ faciliter ‘  is ‘ la vie ‘ in this fixed expression, so no past participle agreement with nous either. 
See attached lesson on the relative pronoun ‘ ce qui ‘ replacing the ‘whole idea’ of the preceding clause here.
https://progress.lawlessfrench.com/revision/grammar/ce-qui-what-which-relative-pronouns