It's obvious that Hachi didn't want to fight Tenryubito and didn't want to provoke him, but he still wanted to free Camie, so he would either be shot by Tenryubito or try to take him down and then Teryubito will call an Admiral who will wipe the floor with those who insulted Tenryubito. Actually he didn't even need to fight Tenryubito as he might call to Headquaters even for being insulted.
About Coby:
1. He didn't know Rokushiki before timeskip! He only knew Soru and quite a weak one. It's only one useless move if you can't back it up with any considerably strong attacking move.
2. Coby didn't know Haki before timeskip. He only awakened CoO Haki and that's all he couldn't even control it. He didn't even know about CoA Haki, so he could neither protect himself with it, nor attack any opponent with it.
Luffy took him down in Water 7 and since Water 7 Coby didn't get any progress on screen till Marineford, where he was still one-shotted by Luffy.
I don't argue that Hachi is considerably weak, but I'm pretty sure that it's obvious that pre-timeskip Coby is way weaker than Hachi if we just watch what are his actual on screen fits, while the same Hachi is a swordsman and Cobi can't protect himself without any protective move like Tekkai against Hachi's 8 swords.