[MANGA SPOILERS] Reason for the Spare

So in the Warriors' backstory chapters, they select SEVEN Eldians to inherit the shifters in Marley's possession. We know now from Chapter 95 that, in this point of the story ~845, briefly before Shiganshina's Grim Reminder, the shifters were RBAMPZ, Grisha, Frieda Reiss, and a Teibar. If Marley definitely didn't possess seven shifters at this time, why did they ultimately reserve Porco Galliard for several years?

Theory: Porco Galliard was planned to take the Warhammer Titan from the Teibar family but things wound up in a different direction.

You can claim that the point of having him on standby would be in anticipation for if one of the other shifters had to die, but that'd take anywhere up to 13 years (in reality, it took either 5-9 years later, presumably the first moment Reiner and co. came back from Paradis. Instead, I want to put out the theory that Marley wanted to own all the shifters as child soldiers in order to dominate the world.

In order to have this goal, all they needed at 845 was to eat The Coordinate, the Attack Titan, and the Warhammer Titan. They might not want to eat the Coordinate due to it's ineffectiveness when not used by a person with Royal Blood, so they would pass on giving it to Porco and wait until they got their hands on a Fritz on the mainland like Dina to gain the Coordinate Warrior. As for the Attack Titan, they still had zero clue where it was so it'd be illogical to plan that far in advance by training an extra kid. However, we know that the 145th king Coordinate lost their footing in the Marley mainland once the Teibar's sided with Marley.

If the Warriors had custody of the Coordinate, there would be no need to keep having the Coordinate's weakness still be independent, so the best move would be to blitz attack the Teibar clan using all the warriors.

However, the alliance had to be prolonged due to RBAM's mission ending up a bust.

TLDR; Why was 6 afraid of 7? Cuz 7 8 9. Why were RBAMPZ afraid of Galliard? Cuz Porco ate the Warhammer.