Well there is only oen thing to say after this, the only appropiate thing to do.
ALL HAIL AINZ OOAL GOWN!!
It's pretty common for many of the last episodes in a serie to suffer delays when it comes to streaming, already happened with a number of other series this season. It happens.
Go towards the OVA section, they mention 3 OVA for overlord.
I have seen that there are going to be at least 3 to 4 OVA released for Overlord, you can them being mentioned in the fall anime 2015 overviews.
Actually the reception to that is warmer than you seem to think and it certainly does a lot better than dangling the promise of a 2nd season to the viewers and then not giving them anything. Trust me.
FMA had this same issue but unlike HotD the makers of the anime went with making an alternative ending and it worked out fine.
Actually what they could have done then is what most anime adaptations do and make up the last 3 episodes themselves, this is not an uncommon sight and many adaptations go with this approach when there isn't any material for them to work with.
Actually that is not true, there was still enough material to get around 8 to 9 more episodes filled with the material left before the manga went on a hiatus. There wasn't a lack of material as they still had more than enough.
The anime for HotD only covers up till ch 17, before it went on a hiatus there were 11 more chapters already published for it.
Glad to see the PV, for a moment I got scared thinking that there might not be a 13th episode, such a shame this eries ends tomorrow. Well here's hoping for a 2nd season tease at the end of the episode.