The idea here is that the magazine uses people's portraits to shed some light on current affairs. The characters always change based on the issue (constantly fluctuating cast of characters). The word "However" indicates the shift which seems to tell us these portraits do less to convey something of the sitters (assuming people or characters) than they do to constitute the picture that represents the complex issue. The issues can be different, so each portrait seems to vary and depends on the type of the issue. Therefore, the first blank should be "type". Anomaly means something unusual - there's no sign of any portraits being unusual. A mirage is out of context.

Now for the blank (3), it is clear that these portraits depict current affairs using characters, and these characters represent people, so "individual" should be the answer. Politicians and celebrities are both tempting, but there's no further evidence support either.

Blank (2) should be easy now, because of the hint in the last sentence, "the subjects within these covers appear more as systematic variations on a standard unit" and the information that the magazine always uses people to depict the national issues. That means the template doesn't change much, and therefore the answer should be "remained the same".

Hope this explanation is clear enough.