How to fix a ruined recovery mode of an Android device?
Qualcomm devices usually have a workaround to flash entire internal partitions like with Oneplus 3 http://forum.xda-developers.com/oneplus-3/how-to/guide-mega-unbrick-guide-hard-bricked-t3405700

Similarly search if Unbrick Guides are available for HP Pro Slate 12. Good luck..