I only logged in because I felt your pain and therefore I too would like to sympathise with you as well. I would like to inform you that I too am having the exact same problem on my 15 Vezel Z (picture attached hopefully). However, mine comes and goes. Sometimes for over a couple of weeks or other times for a couple of days.
I was told by Nazir Electrician (G8 Markaz) the exact same thing that the start stop engine switch needs to be replaced and that the smart key battery is not at fault. I have not replaced it yet.
If I do replace it or you would like to go to the electricians place together, we can do at. Just inbox me your number and we ll fix the problem together. How does that sound?