Wiring – Help me understand complicated junction / switch box — with diagram

Yes this is bad/dangerous because you now have current running on the ground wire at all times. This type of timer requires a seperate neutral (whereas most switches just interrupt the hot) so that the timer can be powered when the switch is in the off position.

In your configuration, the line (power) comes in at the fixture and there is likely a 14/2 cable running to the switch for the sole purpose of interrupting the hot. The other configuration is where the line comes in at the switch which then sends power to the fixture when the switch is closed.

In order for you to use this type of timer, you need a dedicated neutral in the box with the switch. You would need to replace the 14/2 cable with a 14/3 cable (extra conductor) so that you can have a dedicated neutral from the line, and then still switch the hot to control the light.

Depending on the position of the switch and fixture it may be easy enough to pull a new 14/3 cable.

1 white which is hot, and 2 greys which are hot...the switch is on 2 seperate breakers.

This means that:

new switch it only has 1 common hook up, then 3 other connectors on the otherside.

The new switch will NOT work here.

You cannot feed from "2 separate breakers" to "1 common hookup" - you need a different switch that more closely matches the old switches (which look to be 3 separate switches, each with its own connections.)