Hi there

I actually think it will be OK. ATH is not that big and relatively empty most of the time. I have transitted there quite a few times, incl. on connections similar to yours (i.e where I had to go through immigration, back into the departures hall, check in etc). It's doable. Moreover, since it's SkyTeam, DL may even check you through all the way to MXP. And even if not, since your ATH-MXP flight is within the Schengen area, there will be no immigration (passport control) on that leg so you should sail through.