Migrations are about data, not schema. You don't need migrations with a "schemaless" database at the beginning, but as soon as you have data you care about, you're gonna need to ensure some sort of consistency.
I think you should learn the relational model (not SQL). Understand the problems it was trying to solve and how the relational model solves them. Then figure out how that applies to your app. Database in Depth was helpful for me http://www.amazon.com/Database-Depth-Relational-Theory-Pract... .
Yes, you should move back to Postgres. The reason is you can do many more operations on structured data than on structure-less data. You found that out already with the many-to-many thing. There are plenty of other intangibles that will bite you in the ass later.
