The problems of the Feanor and his sons being attributed to a foolish oath they swore may well be mirrored in discovering that Solas is bound by some sort of oath he swore to Mythal.
As for how the elves learned magic under the rule of the Evanuris, the only real example is the one given in the Library. There the elves are seated in an amphitheatre and receiving a lecture in magic from a spirit of learning. That probably is pretty much how it was done. You don't have to be possessed to get your learning from spirits because they can freely interact with you.
Another entry talks about exploring the deeper reaches of the Fade. It says to "put aside ten years for practice, and the next hundred for learning". Is it surprising that the elves considered humans hasty creatures when they can spent a hundred years on an aspect of learning. A human will have been born, raised a family and died during the same period. For human peasants that would be two life times. It is nearly impossible to compare two types of civilisation when the time scales are so different.
The Dalish also say how spells could take decades, even centuries. It seems they were right about this. The entry about exploring the Fade also mentions how "What others have learned will ease your journey". So knowledge would appear to be freely passed on. The construction of the Grand Sollanium would also seem to show that elves were capable of working together in concert in very large numbers in order to complete a project. This enabled them to build some of the elaborate structures built on magic that made up the empire. One wonders if mages in the modern world, in somewhere like Tevinter, would be capable of working together in such co-operative fashion even if they had access to the magic of the Fade as these did.