How does a borg-host correctly fuse with his/her borg, seeing as all the alien-party members "failed" (even Kumi!) their time in the party?
Well, a properly fused human would be like Megumi and Monami and Chisa. Kasumi 'failed' because she fused with the Yellow Knife instead of the Borg that the principals had intended her to fuse with. Yuri 'failed' because she was too scared and didn't fuse all the way. Kumi didn't 'fuse' with her Borg, she turned into a Borg.
What is their goal? Do they intend to fuse with all humans eventually?
I think it's a fight over who will become the most dominant clan and who will get the most hosts. They need to fuse with as much people as possible as fast as possible before all the humans are taken.
The Borgs are less domineering and try to get more willling hosts, rather than the Yellow Knifes which use manipulation, and the Sunflowers which use force.