If you’ve ever had to provide an alibi for anyone, was it just the investigation stage, or did you also testify in court?

Not in a criminal sense.  When in high school a friend of mine had a boyfriend who was Greek.  She isn't.  Her family would not allow her to see him.  She would tell her family she was going to my house and he would pick her up at my house.  No, they didn't get married and live happily ever after.  They grew up and married others and we are all still friends.