When a skill is made legendary, let's say for example archery, would my bow's damage reset as well or will it increase? Same goes for other skills like the magic tree. Let's say I have the ability to summon a thrall (level 100 conjuration) and make the skill legendary, therefore resetting it to 15 or so. Would I still be able to summon the thrall?

You will lose every perk and ability you had in that tree.

The damage loss isn't that bad, at most its like 30 points.

Damn, guess that's not something I'm going to do. It's optional right?

If Your character has enough Magicka, they can cast any Conjuration Spell without Perks in the tree. Twin Summonings require a Perk, but everything else requires having a Large enough Magicka Pool. Potions, Powers and Blessings can be used to boost a characters Magicka.

If I had a character with a very high Sneak Skill, I would have no problems going Legendary on Archery.

I have a character that Dual Wields Bound Swords, I have Legendaried Conjuration twice. The only downside is waiting for Conjurtion to get back high enough to regain the ability to cast Soul Trap, and regain Mystic Binding.

To be clearer, it refunds the perks and resets your skill level to 15. And damage bonus from the level alone will be that of level 15.

Trust me, its not that much. I did it for my 1Handed and hardly noticed the difference.

And do enemies carry on level scaling with you if you go past level 81?

What enemies ever scales past 50?

In short, no, they don't.