Nerwen:
Shifters can teach gifts also, but they're less effective at it, so it takes more time and practice on the part of the student, and it has a lower chance of taking.
This one is correct.
Nerwen:
Normally what happens is that the shifter (or mage, in your case) would have to seek out the spirit that they want to learn from - and whether the spirit says yes or no depends on the asker's worthiness, and also might involve sending the prospective student on a quest. The spirit wouldn't come to the mage to offer.
The party line is that mages can copy effect of the gifts with their Spheres as long as they have enough dots in required spheres. For that matter they require little than just witnessing the effect. Teaching by spirits has little mechanical effect, though as Mage: the Ascension omits the rules for practicing the rotes. Teaching by spirits may help the mage be able to use his foci in ways previously not thought possible and/or to make spells more coincidental, but as far as I know officially spirits cannot teach mages except in some Faustian bargain. They, however, are capable (with Spirit 3 or more) to compel, persuade or bargain with spirits to perform the gifts for them.
Mediums cannot learn gifts, all they can do is invite a spirit to possess them and hope the spirit does what they want. Sorcerers could be what you're looking for. They have ways of creating rituals by directions given by spirits. Unlike gifts, however, such rituals are generally slow to perform and therefore of limited use.
Nerwen:
The best book for it would probably just be the WtA core. There are entire chapters about spirits, the Umbra, gifts, fetishes, etc. I don't recall anything about teaching gifts to non-shifters one way or the other, and I would tend to go with GM's Call (as with any crossover rules). Do what would tell the best story.
If you're interested, Umbra: the Velvet Shadow and Book of Worlds are Umbra sourcebooks for Werewolf and Mage respectively. World of Darkness: Sorcerer and Sorcerer Revised may give you some ideas about mortals and spirits. The Quick and the Dead are Wraith: the Oblivion sourcebook for mediums and sorcerers.