1, It's pretty much expected to crash the game with a negative movement speed... That's the reason there's a minimum movement speed... Mathematically, it's impossible for it to be negative.
2, Try having a dummy spell, like Immolation with 0 mana drained and 0 damage dealt. Change the name to Walk Backwards and make a buff for it called "Walking Backwards" just to make it look nice.
Use every 1 second of the game
Pick every unit in playable map area with buff "Walking Backwards"
-Lock picked unit's facing or head or whatever it can lock to make it face forward to 100 ahead of picked unit towards facing angle of picked unit (I think you can lock to a point. Otherwise, make a dummy unit and make it face that.)
And basically clean it up to prevent leaks. I think that is possible. Then when he walks, it should be backwards... Never tried these functions before.
Anyway, from what I learned recently, almost nothing is impossible to do in the WE. Given you know a lot about JASS and maybe use WEU of course. But most things imaginable is possible. Except of course turning the in-game units into real life objects and then turning those objects into food... Though it would be a great solution to world hunger don't you think?
Anywa, if it doesn't work. I'll see if I have time to help you. I might post here again, I might not. If I do, I will probably help you and have your trigger done, otherwise... I might be too busy.
Honestly right know all my head is filled with is fade filters, bullet movers (basically using triggers to move a custom missle to prevent it from going through units and dealing more damage than it should) Mine is actually more complicated that it has to be... But that was to reduce lag. Checks the area in front of it and it's current location... What a drag...
