Many times when people attempt WILD or a WBTB, a dream starts in the person's immediate surroundings. What you experienced sounds like a FA in which you were lucid. You did the right thing in doing a reality check and it looks like it worked. If you are still not sure, its probably best to try a different reality check since our minds can easily simulate anything it wants. For example, try something that is impossible in waking life. Or see if you can float out of bed. It sounds like you are on the right track for sure!
As far as mantras go, you can have as many as you like and use them as often as you like. Just make sure they retain their efficacy. There has to be an element of belief in the mantra for it to work. That is how it sets the intention. Then later on that intention resurfaces and its power usually is determined by the amount of energy invested in the intention. Hope that helps!