I plan on making a shield+magic user who sneaks around but can also get very up close and personal in combat. Skirmishing playstyle I suppose. I'm aware spells don't recieve sneak bonuses but that doesn't matter with this build.

I'm curious if there would be a reason to level up one-handed weapons, so I can switch from shield+magic to shield+dagger in combat. Or does destruction magic do well enough in close range combat?

I use spells and one handed weapons, and for my shield I just use heal or you can use that spell shield.

You can always hot swap out the heal and sword in the right hand if you wanted to go that route. But I would det level up one handed, because there are some cases you will need to use a sword to kill a mob.
You cant always use magic at least in my 20 hours of gameplay. I think magic was nerfed too much

I'd do sword with magic or dagger with magic.

I have battle axe in one hand, and summons/heal in the other. I FSU on the daily with this combo

Im assuming weapons are better since they scale and magic doesnt