Both are possible and common.

yep. i wrote that too and it wasnt accepted.

It's optional, you can keep it or omit it. But I myself will keep it.

I will keep it here to sound more natural. 个 is often omitted when 每 is before a monosyllable, but not so often in other cases.

Correctness. It's omission is just standard when obvious or during casual conversation.

The continous form is used for describing an activity that is currently ongoing. In this case the activity is taking place repeatedly (every sunday evening).

That could be a correct English sentence if they didn't used to eat noodles every Sunday evening but now they are doing so.

Yeah - I do have the same question - why is it not accepted "They eat noodles every sunday in the evening"? Possibly someone of native speakers here could explain it? Thx ☺️

chow mein is 炒面 (chao mian), fried noodles