I was wondering, does the Magic Resistance Quality apply to the Improved Invisibility/Concealment Power? The spell & power are both magical in nature & add their dice against people seeing/noticing them.

Also, does Magic Resistance offer any help vs Spirits? If a spirit cast a spell at the person, they get the bonus dice per rating... but what if the spirit uses a power or makes an attack against the person with this quality?

Improved Invisibility yes, as this is a spell. Concealment and any other spirit powers, sadly no, but feel free to house rule it.

And actually your example is wrong. Invis is an illusion spell, where everybody has to make a spell resistance test, it does not add dice in any way. Concealment just removes dice from everybody's perception test.

In general Spells are (rulewise) treated completely different from spirit powers. They are as unlike as a toaster and a mountain goat. Note, that spirits (Man and Free) can cast spells.

paranormal powers are strong, some groups have house rules. My favorite is introducing a metamagic that lets you add dice (half Counterspelling or Initiate Grade) to you resistance against these powers.