Is a girl's intimate interaction with young men, short of full-fledged adultery, considered zinā (illicit sexual intercourse), and what are the ways for sincere and swift repentance from such acts?

Fear and hope are both required of a believer. Allah has prescribed repentance for His servants, as He says: "Say, 'O My servants who have transgressed against themselves [by sinning], do not despair of the mercy of Allah. Indeed, Allah forgives all sins. Indeed, it is He who is the Forgiving, the Merciful.'" The sins you have committed are not too great for Allah's forgiveness if you repent with sincere repentance. As for the practices mentioned, they are not adultery that incurs the prescribed punishment, but they are called "zina" (adultery/fornication) in the hadith because they are means that lead to it. This is based on the Prophet's (peace and blessings be upon him) saying: "Indeed, Allah has decreed for the son of Adam his share of zina, which he will inevitably attain. The zina of the eyes is looking, the zina of the ears is listening, the zina of the tongue is speaking, the zina of the hand is touching, the zina of the leg is stepping, and the heart desires and wishes, and the private parts confirm or deny that." Sincere repentance requires abandoning the sin, regretting it, and resolving not to return to it. Among the aids to this are staying away from bad environments and befriending righteous people.