Is it permissible to pray Salatul-Istikhara after Fajr if there isn't enough time? And what is the interpretation of finding a gold ring and repeatedly dreaming of a female colleague after praying Salatul-Istikhara?

You must repent to Allah for being in a relationship with a foreign woman, and sever your ties with her until you marry her lawfully. Salat al-Istikhara is recommended, not obligatory. It can be performed at any time of the night, but it should not be prayed during the forbidden times, such as after Asr or Fajr. What you saw in the dream has no known interpretation, and the one performing Istikhara should proceed with what his heart feels at ease with. It is not a condition for Istikhara to have a vision afterwards. Sleeping through prayer may involve negligence for which one is sinful, or it may not involve negligence, in which case one is not sinful.