Hi @ilo
So you and @robm are both correct, you will get a number of false positives and false negatives on opens, there is a number of ways around this, but they all involve some interaction from the user, for example getting them to click a certain link and tracking that landed page.
In response to part 2, there is a difference between sending transact and marketing messages. From my understanding when you send a marketing message it appends to the header something telling the ISP that you are sending bulk mail and this way they do not penalize you.
I ran into this when wanting to resend an RSS mailer all the time, and found a way to overwrite this setting in mautic. The answer is in this forum post: Disabling Do not Send Marketing Messages to the Same Contact