This question is reminiscent of my proposal to include historical linguistics of biblical languages in the scope of the site. That proposal is over a year old, and I have since come to agree with the then grudgingly accepted answer to reject the idea: this community is not prepared to handle detailed questions about language. There are several very good Hebraists here (I do not follow the Greek stuff), and they provide high quality content, but the community as a whole is not always able to highlight good content through vote counts.
But let's look at your example questions one by one; they serve well to illustrate some points:
Is it a coincidence that עֶרֶב (evening) and עֹרֵב (raven) are spelled the same?
This is essentially an etymological question: do these two words have a common source, and if so, what was the semantic field of that source? It would fit on Linguistics.SE.
It does not fit here, unless you link it to a particular text ("Does the usage of עֹרֵב in Song 5:11 allude to עֶרֶב?").
It also does not fit on Language Learning.SE, because it is not clearly related to language learning.
Is שָׁמַיִם singular, plural, or dual?
This question has been asked on this site — twice, even — and well-received in both cases. Vital in this acceptance is that these questions were linked to specific bible passages.
Linguistics.SE has a close reason that excludes this question (but not the first): "Language-specific grammar and usage questions are off-topic unless primarily concerned with linguistics rather than usage."
I do not know Language Learning.SE, but from their list of topics it seems they are more concerned with language learning an sich rather than language-particular questions. A related question that would probably be on topic there would be "How can I effectively learn to use a three-way number system (singular-dual-plural) when my native language has a two-way system (singular-plural)?", though again, I don't know the LL.SE community.
In Genesis 1:2, why is מְרַחֶ֖פֶת Piel instead of Pual?
This question is similar to the one about שׁמים above. An answer could (should) explain both the morphological and the semantic aspects: (1) could the form be understood as a pual; (2) why is a piel used here? Like the previous question, this belongs here, but not on Linguistics.SE or Language Learning.SE.
