What to do about collaborators & co-authors who fail to understand authorship & plagiarism?
What do you all think?
On the surface, I don't think this is plagiarism.  When a group of people  author a paper, imho the entire group can be considered the originators of the ideas in that paper.  Basic professional etiquette (as well as any co-author situations I've been fortunate to be in) is that each author, when speaking about the ideas, typically says, "We" did such-and-such, or "we" had this idea.  In my experience, many high quality authors, out of modesty, or an abundance of honesty, will often credit their co-authors for many of the ideas.  But unless a person specifically says "this-and-such was my idea" when indeed it wasn't, then I don't think this is plagiarism, but rather as you put it a matter of "basic professional etiquette."

What would provoke 2 co-authors to do this? 
Could be anything from a simple lack of social skills and/or social awareness, to major ego subterfuge and game playing.  Depends on the personalities.

Am I correct in my assessment of the issues of authorship and plagiarism?
As I said above, I personally wouldn't label this plagiarism, but rather a lack of professional etiquette -- unless they outright state that your ideas were their own.  In my opinion, if they use the word "We" (and at some point state the names of who "we" are) then they are entitled to do that, by dint of being co-authors together, even if it was your idea.

What strategies would you recommend for addressing co-authors and collaborators who don't understand what I consider to be basic professional etiquette, but also have a motive that appears to be more devious? 
Discuss with them, as you have, and if that gets you nowhere, chalk it up as a life lesson and avoid working with that person in the future.  Unless it has a true consequence of facts harming your career (in which case you may want to seek legal advice).  But if it's just your pride that has been hurt, because the other person is unprofessional or petty, then remember the phrase, "Every dog has his day."  Move on, a let fate take care of them.

One co author appears to be more sincere and apologetic but the other one, I think the one who submitted the proposal appears cold, and indifferent.  Trust your gut; perhaps avoid all interactions with the "cold and indifferent" one in the future.

One comment that you mentioned but didn't specifically ask about: You mentioned that they didn't reference the unpublished paper, but only the ideas.  Imo this is also unprofessional.  I was taught to reference every source, even if it is unpublished work (just say so).  My graduate adviser impressed this on me years ago, as a strong ethical concept.  I had made some statements in my dissertation that everyone in the field regarded as completely common knowledge (but possibly someone outside of the field would not know), and he made me go back and find the original paper from the 1950s and reference it.  So I have taken it on as an ethical imperative: don't make statements or present ideas without referencing where they came from.  If you agree, then perhaps you may want to convince the co-authors to mention the unpublished work.