Orochimaru wouldn't of beat Itachi even if he was in full health.1. Flamming oodmam rasengan isnt enogh to take out Eight branches. Itachi wouldn't have beat it if he didn't have the sealing sword.
2. I will give you that one if you can give me the episode in which it says that minato had sage mode.
3. Sasuke never technically beat Orochimaru. And Orochimaru was not in full health the second time he faced itachi he was still sickly. The first time Orochimaru just simply underestimated Itachi.