questions on safeguards and audit procedures

Firstly,
For these safeguards ,
1. external review of work done
2. independent review of significant judgement made by the individual

These are the safeguards in which an audit firm may need to perform when they provide both audit and non audit services to a client. But i know that only two of those safeguards will be used on either of audit / non audit service.

In this case, we just assume that only either of the audit or non audit service will need to do independent review / external review ?

But why we don’t do for both audit and non audit ?

Secondly, means if we hire to do an external review, we hire an external specialist and pay the expensive fee just for the safeguard ?

3. when we answer questions (whether section A or B ) Do we need to know whether it’s immaterial or trivial ?

when it’s not material, can we just simply say immaterial ? and disregard trivial ? if this doesn’t really eat up the marks ?

4. in writing an answer for the audit procedure, is there a difference between inspecting the purchase agreement ( for e.g to confirm the brand cost) and the reviewing the purchase agreement

or inspect the board minutes ( eg to confirm the approval of acquisition ) and reviewing the board minutes