Hi Ben,
You could do the first, plug the Equalizer 3D into the recevier and split the power. You really do not need to use a 3D and its aux power input on that size plane, I would recommend a Equalizer II and plugging the other power lead into the receiver using a small Y (I think 3" Ys are available) on throttle. I would not recommend using the regulator you have with the PowerExpander Sport the way you propose. The servo outputs are not meant to handle more current than the servo requires. You could do this but you would get extra voltage drop going into the PowerExpander.
Please let me know if you have any other questions.