Ok 1) pants is already plural, as is scissors. Luggage is a collective noun, which uses singular constuction no matter amount (it does have a plural of luggages, but who cares) Chalk is chalks, but moose, sheep, and aircraft do not change with plurals.
2) I see your point 3) I think German has more irregular plurals
If there’s no definite article (i.e. masculine would mean singular and feminine plural) and you’re going on the sound of the initial vowel alone, the Ä sounds like /e/ in bed for the plural and the plain A sounds like the /a/ in father for the singular (in a general American accent).