Can Zoro during his formative years at the Isshin Dojo defeat Orochi in both of his forms?
VS
Scenarios
1. Kid Zoro with 2 unnamed katanas vs Orochi in his human form using his unnamed katana
2. Kid Zoro with Enma and Ame No Habakiri vs Orochi in his Yamato-No-Orochi form
For scenario 1, I can see Kid Zoro taking it pretty handily, as Orochi displayed no proper use of swordplay, nor is he regarded as a strong fighter in his human form.
Scenario 2 is tricky, as we don't know what attributes Orochi's df form provides him. It could be instant regeneration like your hydra, or a tough hide.

VS


Scenarios
1. Kid Zoro with 2 unnamed katanas vs Orochi in his human form using his unnamed katana
2. Kid Zoro with Enma and Ame No Habakiri vs Orochi in his Yamato-No-Orochi form
For scenario 1, I can see Kid Zoro taking it pretty handily, as Orochi displayed no proper use of swordplay, nor is he regarded as a strong fighter in his human form.
Scenario 2 is tricky, as we don't know what attributes Orochi's df form provides him. It could be instant regeneration like your hydra, or a tough hide.