I don't think the back plate or the helmet are "misshapen". The original pieces from the original films (Screen used-Lucas Archive) are all misshapen and a-symmetrical. The original sculptures where pretty poor when it came to symmetry. The original patterns also had plywood end-grain and chips when the parts where originally vacuum formed and all of these flaws had to be reproduced exactly, per direction of LF.
As far as the hoses, again, originals where not superior quality and the hoses are just "screwed into the holes". But I have less knowledge of the production of the helmets.