While wielding a daedric dagger, having 100 one handed(all perks) and 100 sneak(all perks), the daedric dagger's base damage showed as 33. From the dagger's sneak attack(only 15x damage) it caused around 500 damage. Then I applied a power attack and it caused 1,000 damage. Then my orc used berzerk and a single strike caused 2,000 damage.
I could do some more testing (like getting shroud gloves, dual wielding with a power attack doesnt do a single hit unfortunately), and having bonus one handed damage enchants(it modifies the displayed base damage btw), but I think this is enough to show that multipliers stack (15x + 2x = 30x + 2x = 60x) instead of combine(15x + 2x = 16x + 2x = 17x).
Weapon skills (Archery, One-handed, Two-handed) : skill increases damage done. (0.5% per level. 50% increase at level 100)
Armor skills (Light Armor, Heavy Armor) : skill improves armor rating mildly (perks are where it's really at though). (0.5% per level. 50% increase at level 100)
Magic skills (Alteration, Conjuration, Destruction, Restoration, Illusion) : skill reduces magicka cost. Skill also unlocks new spells on spell vendors at 25, 50, 75 and 100. (about 41% reduction at level 100)
Lockpicking : skill broadens the target zone.
Pickpocketing : skill increases success chance to pickpocket
Speech : skill improves prices when buying and selling, and increases chance of success for persuade and intimidate.
Sneak : skill reduces detection radius for sight and sound.
Alchemy : skill improves magnitude of potion/poison effects.
Enchanting : skill improves magnitude of enchanting effects.
Smithing : skill improves armor rating of upgrading armor and weapon damage when upgrading. Does not improve item creation at a forge, only upgrading is affected by skill.
Block : skill increases percent of damage blocked.
Best Answer
Legendary status was implemented to allow resetting skills. Side effects of this was that you could level above level 81.
In Skyrim some enemies scale with your level while others do not. Wolves for example will stay weak the whole game while higher tier enemies will be 'difficult' the entire game.
Before resetting your main skill you should test out how strong the difference is. Just save the game and do a dungeon or two.
You will loose all benefits of this skill tree, meaning damage will drop, special attacks are missing.
If you manage to play this way you will be rewarded however. As you use one handed anyway you will level it without really noticing. Also you will gain extra experience from these level ups meaning you increase you total level resulting in more stats and skill points.