Some time ago, there was a post on LibreElec and Collabora Twitter about work on a driver for Mali T760 and newer. If the driver is created and working, will it be possible to hardware decode without HEVC, just using this driver ( something like hwdec=auto in mpv) and solve the video rewind issues? - I don't quite understand how currently HEVC uses GPU hardware decoding if we don't have drivers.
No. In the x86 world the "Graphics Card" sort of combines everything into one device. In the ARM world GPU and VDEC are totally separate things. In an Amlogic box the Mali GPU (Panfrost or Lima drivers which are upstream and work great) accelerates rendering of the Kodi GUI via OpenGLES (squares and triangles are being drawn on the screen via maths) and an IP block on the core SoC (S905 etc.) decodes video frames for rasterising to a DRM (digital rendering manager) device like an HDMI tranceiver.
Fixing HEVC is going to be a lot of work as the older and newer hardware generations are quite different. I'm not going to go into too many details just yet, but there is a plan of sorts forming on how to get some of the outstandinig issues resolved (and who will do the work). It's unlikely to be quick though, so we have to be a little patient.