As you noticed, Destruction is pretty straightforward. Each cast and while continued casting, you will increase skill, but this has a caveat: only if you are attacking someone. If you're fizzling your spell in the middle of town, it's not going to increase it.
Alchemy is also straightforward: each potion created will increase your skill. As dpatchery notes, eating ingredients for effect determination will also increase your skill by a small amount.
With Enchanting, each enchant (or disenchant, as LessPop_MoreFizz notes) as will increase skill. dpatchery also notes that you can increase your skill by recharging items with soul gems.
Restoration, namely the healing spells, only increase skill if your health is below its maximum. The others only work if you're in combat or near hostile enemies: this includes spells like Steadfast Ward.
For summoning spells (essentially most things in the Conjuration school), you won't get a skill up for the summon until you enter the range of hostile mobs and, in the case of summoned creatures (like the Atronach and the Familiar), they do some damage. With bound weapons, entering range of hostile mobs is all that's necessary to gain Conjuration skill; however, using them in combat will increase their appropriate weapon skill, not Conjuration.
Alteration and Illusion spells that are targeted towards messing with hostile mobs need hostile mobs within range to skill up. Buffs, like Oakflesh, don't increase skill without hostility as well. Others, like Candlelight and Muffle, are recastable without hostility but only increase skill by a small amount.
Since you can't use the developer console it might very well be unsolvable, but they might correct it in a new patch.
Few ideas I've got:
Try to become a vampire, when you are standing in sunlight, all your attributes get permanently (until it is night or you are indoors) decreased, the magnitude depends on which stage you are.
There is an effect, accessible only by using alchemy, that permanently lowers health. It is not the damage one, but Ravage should do it (if I understand it correctly). Although I don't really know whether the potions you create with both positive and negative effects are classified as potions or poisons.
Both of these effects are reversible and might very well not be of any good, but you can at least try.
I am sorry I couldn't provide anything better.
Best Answer
When in Skuldafn:
So even if you could exit Sovngarde back, you would not be able to reach skyrim.
Your best option at this point is probably to load an older saved game, before going to Skuldafn.