So some bullet points to help you understand, based on info you would have by now that's not given in the show ever.
•You nailed the goal, it was also a test for Lupis Regina to see if she learned, if she would interfere. The biggest reason *this* was the way Ainz did it, my guess not info we're given, is because last time he just *gave* them a magic item it got Enri in trouble. Now they can say "we won it in a fight against a powerful monster who attacked our village!" sounds more stomachable.
•That *was* in fact the "Giant" zombified, that's why they commented on it being so zombie like. Ainz wanted them to fight it, to test them, but not for it to be so powerful that they couldn't win, which is how it would be if it were alive cause of how strong a trolls regeneration is. So he just made it a zombie, gave it back the sword and sent it at em
This show has a *lot* of information in it and that's a DROP compared to whats in the LN so always feel free to ask for LN info if you want (or just read the ln if you enjoy reading) I'll give what I can without spoiling anything!