Question on Upgrades

Let's Say I have a clean bow with no mods....
Can i upgrade it with a grip AND a string? or only one will work?
Also can i put minor, major, and sup vigor on armor and get benefit from all, or only the highest will work?

Yes you can attach a grip and a string but if you try to salvage it you will only get one back or crafting materials.

You only get the benfit of the higher Rune.

To clarify, you cannot use more than one of the same rune no matter what the level. For example, I have the following.

Major Rune of Marksmanship
Major Rune of Expertise
Major Rune of Wilderness Survival
Superior Rune of Vigor