Hello and welcome
Ok, to answer your questions.
1) No, unless your betta is sick and needs medication, just a good water conditioner should be fine.
2) It is possible that you stirred up gunk from the gravel which released ammonia into the tank causing a reading to appear. It is also possible that you have ammonia in your water-I would test it. Also, do you use test strips or liquid tests? Test strips tend to be very inacurate and could cause these inconsistancies.
3) 7.6 should be fine for a betta (that's what mine are in). It is nearly impossible to lower pH because water with high pH usually has a high natural buffer, meaning that the pH will "bounce" back if you try to change it. Also, manually adjusting pH each water change is subject to error, and a fluctuating pH is worse than a stable one that is not completely "perfect." That being said you can use driftwood or indian almond leaves to release tannins into the water that bettas enjoy and that slightly soften the water.
Feel free to ask any other questions that you may have.