On this site is the link to Spain Original on htcmania from John Miller, ROM Rom Cerberus V1&V2 - Android 6 C432B594 (Dual Sim) - 03 Marzo 2017.
You can translate it with chrome.
Yes, MultiWindow is working, but not so good as in Android 7/7.1. But it's better than nothing.
It's your decission to flash it. Do what you want Do a backup
And don't forget: super mario run not working at all the parts on this rom. Sadly
And don't forget, rom is prerooted. You can flash it and you are fine. Hope you get it work.