Welcome to soap making & to the forum
Just to let you know, adding extra oils or butters like shea butter will decrease your bubbles and lather of your soap. But if you'd still like to add it, I'd stay under 1/2 an ounce of shea butter per pound of soap base.
As for the lemongrass essential oil, I'd not go above 0.36 ounces of essential oil per pound of your soap base. It's an essential oil that sticks well, so you wouldn't want to over scent it.
Brambleberry has a nifty little fragrance calculator that may come in handy for you, you could check out what they suggest for lemongrass EO.
http://www.brambleberry.com/Pages/Fragr ... lator.aspxI melt MP in a double boiler all the time, so you should be fine