ok. clean install Win7, re-install all essential drivers except for Ricoh Camera driver and Alps Pointing Device driver (apfiltr.sys). Looks like I'll be postponing the modded IDT audio projects until mid-October or early November. around that time, Microsoft should have Windows 10 (Wave 2) or "Threshold" Wave 2 ready, which should have some improved compatibility with older hardware & devices.
found another 6504 driver (date is 7/02/2015) from HP recently posted there (sp72373) http://ftp.hp.com/pub/softpaq/sp72001-72500/sp72373.exe release notes for sp72373 posted here. for HP Z1 G2 Touch Workstation PCs with Windows 10 x64 only hardware IDs supported by this 6504 driver: HDAUDIO\FUNC_01&VEN_111D&DEV_76E0&SUBSYS_103C3561