Obi-Wan knew Anakin was behind everything (Short of Palpatine), and there was a possibility of things resulting in violence, but as far as he was concerned there might have still been a chance for him, master, friend, brother, and mentor, to turn him back towards the light.

This isn't true. We see very quickly that Obi Wan has given up on saving Anakin. That's half of what his character says in the OT... that Vader destroyed Anakin

If you play close attention to the choreograpy, you can actually see the detail that Lucas put into showing the relative skill that the two had against one another in battle. The entire battle you see Obi-Wan fighting on the backfoot, and Anakin is constantly in the spotlight holding his own and shoving Obi-Wan back.

I did pay close attention. For example, I payed attention when in ROTJ it is established that an enraged Luke, who has practically ZERO lightsaber training, was able to overwhelm Vader. Anger and hatred are powerful and it isn't always about skill. Obi wan is absolutely overwhelmed with emotion... emotion that we never once see in the fight.

I'd like to believe, sincerely, that killing your best friend, and figurative brother, is significantly easier said and viewed from a background party perspective than actually going through with it.

Which I would agree with... if he hadn't quite explicitly stated that in his view, Anakin was dead immediately before leaving. "You were my brother... I loved you". Anakin Skywalker is dead in his mind and that doesn't change through the entire OT. His friendship is repudiated onscreen.

I'm pretty sure he didn't anticipate Anakin's smouldering body to survive the damage he received while being cooked to a crisp from molten lava, let alone the return of Palpatine to Mustafar.

A moment ago, he couldn't execute his brother... now he's willing to let him broil alive just too avoid getting his hands dirty?