I will be referencing information I got from these sources:
Here it states that Batman had actually killed Scarecrow using his own scythe to decapitate him. So here is my idea for Nightcrow's(?) lore based on the kit you have provided:
Scarecrow's fear toxin is gaseous in form so I think that Nightmare Scarecrow is actually composed of his own fear toxin, the vials he carried was crushed in some way, perhaps during Nightbat's egress from the scene, releasing the toxin into the air. While Jonathan Crane was dead, his spirit was unsatisfied, having felt like he was prevented from enacting the revenge he longed for. Looking for a new form to take, he imposes his will on the released gas, coalescing it into a new "body." Gas normally isn't solid of course but it is Scarecrow's spirit that keeps it together and serviceably corporeal. He takes up his mask from his severed head and forms armor out of materials lying nearby (for maximum spook I was thinking that this event could take place in a graveyard and his vials were crushed by a fallen gravestone, this gravestone providing the material he would use for the armor but I do not have access to the source material at this time so it might clash). He also takes up his sickle once again, this being the source of his basic attacks.
The major aspect of Scarecrow of course is the hallucinations caused by the fear toxin. In his new ghostly form he no longer has to rely on having people ingest the compound to hallucinate, he can create illusions for them himself by imposing his supernatural will. This is the source of the clones described in the kit. In this new form he has made for himself he seeks to continue his campaign for revenge, not just on the jocks who had bullied him in school but on batman himself for getting in his way.
That's what I got, hopefully it is line with your vision of the character. Now on to the kit analysis.