I'm sure it'll be fine! They normally do an RNS on the forearm but they can do them anywhere. The RNS is a little painful to do on the face and you do have the right to say no to that.
They send a signal in a repetitive manner, thus the name.
For example, zap . . . zap . . . zap . . . zap . . . zap. And then again like that. The time in between "sets" of zaps may increase in intensity or decrease in the interval between them. Ask them to tell you what they are doing - which they usually do - so that you are prepared for it.
The doctor may want to do an EMG if they find something in the RNS. They want to be thorough, thus the long appt. time. That doesn't necessarily mean it'll take that long.
An RNS is not specific, so they look at the results in combination with the clinical exam, other tests done and response to any treatments. That would be the same for an EMG or SFEMG.
Just take one thing at a time. Tell them how you're doing. Most techs are very nice! Let us know how it goes.