"Er wohnt immer alleine und hilft den Menschen."

[VaterGut]

could someone explain to me the additional -e- on allein-e here, please?

[dimitar]

alleine seems to be colloquial, allein is the correct form.

[koyunlar]

Two questions:

Is MEnschen singular or plural here?

Is Menschen in dative or accusative case here?

I thought it should be dative (jmdm (dat) helfen = to help someone)

When it said I got it wrong, the explanation was "In accusative case, use "den" for masculine nouns like "Menschen""

So I am confused about two of these (singular vs. plural, akk. vs dat.)

[olimo]

"Helfen" indeed requires dative case, so it is definitely dative plural rather than accusative singular. Singular dative would be "dem Menschen".

The explanation you got is just wrong. These explanations are computer-generated, I guess this hint is just shown when you type "dem" instead of "den". It should be reported.