I still think that Glaurung would still feel like part of the season if he becomes embodied in Episode 9 instead of Episode 3. If the project is not a reaction to the failure of the Dagor Aglareb, then we're depicting Morgoth as reacting to that battle by completely failing to think up a new plan. After the battle he just stops trying for 400 years? He needs to react to the Dagor Aglareb in a proactive way, which is what the Dragon project would show.
If Glaurung is an entirely new plot point starting in E9, Episodes 1-8 will be one arc which will end, with E9 starting a new arc which will end in E13.
Also, you are implying that just because the orcs fail in the Dagor Aglareb, Morgoth must come up with an entirely new stratagem at that point. This is not true. Just because Glaurung exists prior to the battle does not mean that Morgoth isn't changing his plays up. Glaurung is not utilized in the battle, and therefore is STILL a new tool of war. Morgoth is not standing still. If anything, he is standing still if he doesn't attempt anything new between E1 and E4.
Confrontations with the minimally trained Sindar go poorly, but confrontations with the Noldor are flat-out disastrous. This is not due to the presence of the sun, since both of these confrontations predate it. Sure, trolls tip the scales to a degree, but Morgoth never has them in the kind of numbers he would need to field an army of trolls alone. This implies that there is something limited about their production.
If Glaurung has to be visible from his beginning I would much rather not make him something small and harmless-looking. Even newly-embodied, his body should be large and menacing, crocodile-sized. Making him into a small pet has the danger of trivializing him. Likewise, depicting him as something so incosequential that Morgoth abandons the project for several centuries would also trivialize him.
I don't disagree on this point, nor the suggestion that firebreathing be reserved for E13. I'd hate for Glaurung to appear to be some sort of Salicious Crumb character even temporarily.
@Haakon I realize that may not be your intent, but I do fear that it may be the result.
I do feel that we can find a way, however, to portray proto-Glaurung in a way that does not trivialize him and yet does not give him away too much.