Insomnia, frequent waking, very heavy snoring, unrefreshing sleep, fatigue every day. Sleep study confirms no sleep apnea present. What could it be?

You may suffer from. Upper airway resistance syndrom, most of the time this diagnosis is missed by sleep studies. Have them compare your rdi to ahi if it is almost double, you have issue, even though your ahi is very low and your oxygen saturation is above 90%.
Snoring. Was this an overnight, witnessed sleep study? Snoring solutions: avoid sleeping on your back ( sew a tennis ball into a pocket on the back of a t-shirt) lose weight see and ENT to be sure there aren't any nasal obstructions take medication for allergies use breathe rite strips have a dentist custom make an oral appliance.
UARS. In patients that snore and have frequent awakenings with nonrestorative sleep, the diagnosis is often uars (upper airway resistance syndrome) which is in the similar spectrum to apnea. Often devices such as snore guards (one such is snore rx) and treating nasal congestion (theraventsnoring, breathright, nasal sprays) can be helpful. See sleep specialist if not getting answers you need.