Kodi only detects output config at startup so if you connect to the TV after boot it didn't receive necessary info. You can run "getedid create" to cache/force the EDID data from the HDMI connection, then the box will never see HDMI state changes that might be upsetting things.
There's not much we can do locally if suspend/wake doesn't work; it's all about the upstream kernel and what that inherently supports on your device. The challenge with mac mini's has always been that there's no BIOS that power states can be tweaked from, and no upstream kernel developers are using decade-old Apple hardware to validate Linux support so regressions can creep in over time (or things never worked in the first place). Your best bet is generall Googling .. if there's a patch we can add or a missing kernel module to enable; we can probably do that, but nobody on the team has old mac kit for testing so we're shooting blind on the problem. You can also report the issue to upstream kernel maintainers and see what they suggest. If you do, feel free to cc me (chewitt@ the LE domain). The power-management and ACPI mailing lists are probably the ones to target for Qs:
Linux power management - Patchwork
Linux ACPI - Patchwork