ai – "not enough sense to come in out of the–"
eig – what Roger Daltrey wanted Love to do o'er him. This is indicative of ruling or being in control.
ei – this one refers to the little straps used to control a horse while riding it.
The fact that both of the second definitions refer to being in control of something has led to confusion about which spelling to use in the phrases "free rein," "full rein," and "rein in." These phrases describe the amount of control a person has over a situation. But they don't describe that control directly; they describe the application of that control. They speak to the person applying the control, not the person receiving it. It's not that you receive the ability to reign over a situation, it's that you are using figurative reins to oversee the amount of control a person has. "Rein" is the correct spelling for each of those phrases, even if I've done a terrible job explaining it.