The technique you describe when building the helm is the same one I used,
Keeping the lower part of the roman helm in tact.
I do not thing that halberd is period for early Celts.
I am not an expert on this period, but I would say no.
Halberds did not start looking like that until the later 17oos.
Even early period halberds from the Middle Ages looked nothing like these.
So I would say no,
besides, I think with Gauls/Celts it is best to keep them very simple.
They would not have made something to look like that unless it were extremely practical.