Is the 'he' before the 'haluavat' necessary? I thought it is already implied by the conjugation from 'haluta' to 'haluavat'.
First and second person pronouns can be omitted, third person is the odd one out, and the pronoun is required.
In a reply to a question, it could be left out, like
- Haluavatko he syödä mämmiä?
(or simply Kyllä)
Why mämmiä and no mämmi if "some" is not included (or a number)
In the English sentence, mämmi is still an uncountable noun, even without the word "some" to emphasize that it's uncountable.
An uncountable noun needs to be in the partitive when it's the object of the verb.