Just wanna make sure I'm interpreting this interaction correctly.

Red She-Hulk uses her Mighty Big-A$$ Sword rolling a 4-6 against a figure with Mastermind and then that damage from B/C/F is transferred to an adjacent character with a lower point value then the character. What happens to the additional damage from Mighty Big-A$$ Sword's trigger? Would the effect also be transferred along with the damage or would the character using Mastermind still be considered the "Target" of the attack.

The target stays the same, so if they MM to a lower point figure they would take the damage. Then you would choose an adjacent figure to the target that is not adjacent to her to deal the damage to after the action resolves.

So I could basically choose to deal the MBAS damage to the figure who had damage already transferred to it by Mastermind if I choose to OR I could choose a new character who is not adjacent to RSH but still adjacent to the figure with Mastermind. That all sound correct to you?

Yeah, the target of your attack never changed, and nothing in the power prevents you from choosing someone that has been dealt damage by this power either.