The first three (Marisa, Alice and Reimu) were picking on Yuka and then when Yuka was backed into a corner, a bunch of Wriggles came and saved her. Yuka, with a heart of gold, let them live and decided to take care of the three abusive Yukkuris now that they had been damaged so badly are useless to the herd (just like they said Yuka was).
...Or she means to slowly punish them. I dunno. You can't tell with a Yuka. I'd say option A, though.
i think it depends on the attitude of the particular yuuka, some yuukas are just too nice, some others are hardened yet averagely benevolent yukkuris that know hot to defend themselves after living a life of outcasts because of the yukkuris in her zone being a bunch of racists that try to take over her plantations.
Essentially boiling down to individual circumstances. Most Yuukas are stronger than the average yukkuri because of their lifestyle but aren't born that way; either they adapt or die.
Or something like that. Yukkuris seem like they'd follow the rules of Lamarckism evolution. Personal canon I guess.
I think Yuka is said to be stronger because they work all day digging and planting stuff while the yukkuris just take it easy. Stronger than the three stooges, but weaker than an alice in heat is what is usually said.
At first, my train of thought was just like you, "why did she give credit to all of her enemies like that. Is she a stupid yu or is she trying to ingratiate them?" and then just as harouki said, I looked closely at Yuuka and the trio, I noticed that Yuuka had a very sadistic smile. In contrast, the other three looked so terrified and perplexed, so I immediately got what she was trying to do. Smart move. Pray for those three.