I've been trying to figure out a "plan" for a character that combines the elements of a mage and an assassin. Basically, the use of Enchanting, Alteration, Illusion, and Conjuration in conjunction with Sneak, One-Handed, and Alchemy. The problem I find is the right way to progress my character so when the fights do come, I don't feel totally useless when forced fights come.

I know Sneak to level 50 and going a long the right side of the tree is a priority, yet I am unsure how to progress beyond level 10 or 11. Should I focus on leveling Conjuration, then max out Enchanting? Should I get Illusion to 75 then max out One-Handed for the high damaging sneak attacks/invisibility? When should I level Alteration and Alchemy? Sorry if I sound a little OCD, but I want to at least be able to handle forced fights when stealth is not an option. Your input would be highly appreciated.

I'd focus first on sneak (I like getting pretty much at least one of everything), a tad bit of one-handed (Armsman and the Dual Wield perks specifically), and Illusion (Quiet Casting should be priority here). These will be your bread and butter for 1. Avoiding Conflict and backstabbing people, 2. Taking care of yourself in pitched battle, 3. Getting out of hairy situations when it's best to flee.

Alchemy and enchanting you can level as you go. Conjuration will be useful for Soul Trap and Bound Bow, but I don't think Necromancy or Atronachs are particularly Assassiny.

I HIGHLY recommend Pickpocketing (Everything but Keymaster usually find their way into my build), especially the Poisoned Perk, as it allows you to complete your assassinations with a greater variety and more interesting ways (lift a weapon off someone then make them attack with a frenzy poison never gets old, nor does paralysis).

Also, OP, I'd highly suggest picking 3 of those Skill trees to focus most of your efforts on. From looking at your list of Skill trees, those ever-so-precious Perk points are going to be very spread out, and not going very far up any one tree...

Focus on three core Skill trees, and only put Perks points into the others as you have need of them.

now, you don't have to do it this way, but I think it's sound advice in my opinion.