Vengeance most Fowl has a massive plot hole...(SPOILERS)

I feel like Mauler watching DP3 everyone says it was amazing and they enjoyed it, and here i am going "i...was disappointed" I'm probably going to make a post of all the problems the movie has but there's one massive glaring one that came to my mind 5 minutes ago.

Ok, so Feathers has taken over Nor-bot and commanded him (or rather sent instructions) for him to build 20 more copies- steal a bunch of stuff - build a submarine - come get him and take him to Wallaces house where the blue diamond he hid all those years ago is.....do you see the plot hole here?

Feathers KNOWS where the blue diamond is hidden, as he hid it in the teapot all those years ago (some freaking how) he has full control to input commands the Nor-bot follows to the letter, he had this all planned out and built a machine to put it into action....

WHY DIDN'T HE TELL THE ROBOTS TO BRING THE TEAPOT/DIAMOND WITH THEM?

If he told them after building the sub to find the teapot and bring it to him, he would have escaped on the sub - Gromit can't get to him, due to being underground, he would then sail off to the sea and be free, meanwhile wallace and gromit would be locked up.

It's very clear ALL he cares about is the diamond, he spares gromit later when he hands the "diamond" over, his focus isn't with WAG the only reason he trapped them was cos they found out.

I get the film wouldn't have the ending it had but, blooming heck did no-one think about this when writing the script?