Why does Kieran MacDuff become first officer rather than captain?

Question

In the Star Trek: The Next Generation episode "Conundrum", the entire Enterprise crew experiences amnesia, and a new crew member appears: Commander Kieran MacDuff, the first officer. Given that

the MacDuff is actually an alien intruder from a race that has artificially induced the aforementioned amnesia and modified the ship's memory to add MacDuff to the crew manifest, with the aim of tricking the Enterprise into wiping out their enemies,

I have to wonder — why did he become first officer rather than captain? (From an in-universe standpoint, I mean.)

Answer 1

At the end of the episode, "MacDuff" strongly insists that Captain Picard gives the order to fire. He wanted a real Starfleet captain to issue the order (attack the station). Only after that failed did he try to do it himself.

There is no information as to what plans he had for after the attack, so my speculation is that it would play somehow to his advantage, perhaps a way for him to disappear without many(any) questions and/or get more time to run, by creating more confusion.

Starfleet Command wouldn't at first realize why the attack happened if the Enterprise were still be under Picard's command, but if MacDuff were captain, he would disappear immediately and the ruse would be clear then.

Answer 2

It might also have been easier to insert someone into the middle of a hierarchy than it would be to insert into the top. His rank, Executive Officer (not First Officer from the OP), was not something common in the series. This could be seen as an easier manipulation as you are shifting around fewer changes in memory and in the computer.