There is no restriction that the illusion cannot occupy the same space as a creature (it is not a creature itself and the text does not mention it). You can also create the image of a creature you can see easily and slight modifications, like the lack of wounds should be within the capabilities of the spell. The only hiccup could be with moving the illusion to overlap with the creature at all times. The spell does not state that the image can move by itself, only says that:
.. you can use your action to cause the image to move to any other spot within range. As the image changes location, you can alter its appearance so that
its movements appear natural for the image. (PHB 258)
The precise details will be up to the DM, but it seems reasonable to rule that if you spend your action every turn on moving the illusion, it can stay overlapped with the creature.
Unless you are a high enough level wizard of the school of illusion, you cannot change the image after you have cast it. You cannot make only some wounds appear.
You can only cover things up, you cannot make anything disappear. Like possible sprays of blood from a really damaging slashing attack.
About the image giving orders
Unless the real creature is mute and/or deaf, he will also hear the things you make "him" say. It might give away your trickery sooner. Also, he can just override your "orders" with his own. You might create some confusion in the ranks, but then some of them will surely make investigation checks to clear it up.