What to do about not receiving credit in a paper (lack of acknowledgment)
In my opinion, aknowledgements are not important enough to get bothered about. Very few people would have even noticed the acknowledgement, and your career would not have been helped in any measurable way.

But at least in your second example, which was more more of an "idea for a paper" than a "helpful comment," I do consider it a breach of etiquette for your collaborator to write the paper himself without offering to involve you as a coauthor. Perhaps in the future, you could phrase such helpful emails as more explicit invitations to collaborate. That way you won't have to be stingy with your ideas, but you'll still get credit.