Yes but i'm not because I am saying that if we dismiss an external creator and spontaneous existence we are left with an eternal universe which contains both the necessary and the contingent.
No - if the universe is itself eternal then I doubt it 'contains' anything that is also eternal, where everything in it that we can know is contingent.
I cannot be making the fallacy of composition because I am not contending that the universe is just the contingent parts
If you recognise that parts of the universe are contingent and then you conclude that the whole universe is contingent, which you would have do if you also claimed a 'necessary' that is external to the universe (the non spacio temporal you mentioned - like 'God'), then you could well be committing the fallacy of composition by denying that the universe could not be an eternal but your choice of 'God' must be.
I don't know the name of the fallacy but if you are claiming you can have contingency without necessity then you are in the wrong.
I'll always concede the possibility that I might be wrong, and I can see that the dichotomy of contingency vs necessity looks nailed-on and in a sense comforting for those who would like of First Cause called 'God'. However, there remains the issue of how to even conceptualise something that is eternal or a necessary is "non spacio temporal", and of course it is important to recognise that there may be some relevant 'unknown unknowns' down the road somewhere that we might one day discover, or maybe not discover, that in some ways extend beyond the assumption that the only options are contingency vs necessity.
Perhaps we should progress on this at some future point - say 500 years or so from now.