What are the legal implications if I make a fake Instagram account with my girlfriend's name and try to make a certain guy confess that they had sex?
An act doesn't need to be illegal for it to be used later to support a whole range of allegations, charges & consequences.... Think grounds for a domestic violence order, (yes accessing a phone, & setting up a fake account are valid grounds yo get an order) along with the consequences that may come with it, including a criminal conviction & possible jail if you contravene a condition of the order..... That's not even getting into the consequences that could flow from you actually confronting anybody as a result... Things can escalate very quickly..
What you are contemplating is a recipe for disaster.. for YOU.