Wow, so much information in one place. Just what I was looking for.
We actually got our Quaker a few months ago, and his personallity has improved with us.
Many of the points you made, described him specifically. Hense, the name Sassy. But, I learned that he likes routine. He knows when it is time to come out, and yes - he is so protective of his cage, we actually have to wear a glove for him to step-up on. He loves coming out. He knows his "glove" and when he sees it, he will meet you at the door. When he doesn't get out when he knows it is time, he will scream to remind us.
He loves his jungle gym and knows he gets nutriberries when he is playing on it. He doesn't get nippy when he out.
So, as long as we stick to his routine, I believe him to be as sweet as my conures.