There are neither exact nor consistent ways to determine where primary ends and secondary begins -- or where secondary ends and tertiary begins. The definitions always seem to depend on who's using the words.
I guess D has a short attention span for secondary relationships. He can invest some extra umph into them at the very beginning, but then soon seems to lose all interest. I think the kind of partner you're looking for would be more long-term and committed -- even if the relationship was secondary.
So, you are probably making the right move to just forget about D as much as you can. I get the impression that he has already forgotten about you.
Yes. Pretty much what you said in the bolded. That was what was described to me but not what has panned out. So onward and upward.