Those questions, and others, may be things you think about for years. It's a process, I think.
The couple I'm involved with have been poly for about a decade, and we're all in our "mature" years, ie over 50. Still, despite long and solid experience L is having trouble understanding how D's current girlfriend fits into the relationship -- she feels that the GF doesn't really want to acknowledge that L is the wife and always will be.
So in a sense L and D are still facing things they have to work out.
And even with these very stable, thoughtful, and generous partners I had considerable emotional whipsawing in the first few months. It's to be expected! Very deep and primitive drives come into play.
I think poly is aesthetically and ethically beautiful, but as many people on the forum write over and over, it isn't necessarily easy. (But neither is monogamy.)