Getting engaged is something serious. It means something serious is going on. If you propose to someone it generally means you love the person deeply.
Thus I cannot begin to understand why Zach whom last we saw in love with Ella and whom we last heard of when Ella said HE proposed - meaning at that point he still loved her - why as an engaged man, why after sending out invitations to the wedding, why Zach’s mind would go off like that and switch on ‘Oh-let’s-hit-on-that-teenage-girl-whom-I-never-saw-before’.
This is completely out of character and I don’t understand the point of making a fuss about a wedding back in Season 4 and have the fans cheer only to ruin it THAT way in Season 5.
The only explanation I will accept now is that Zach has the twin. Otherwise I’ll just pretend this never happened and Ella and Zach happily married back in Austria.