Would "it is from us" also be acceptable?
No, English does not 'from' to indicate possession.
Think so, let's report
Both have same meanings but "it is ours" means that "they" feel more reluctant to give what belongs to them" rather than "it is from us". I think this verse depend on the situation? Please cmiiw
why is it van ons, not just ons?
"ons" would then translate to "us" ("It is us").
"is van ons" practically means "belongs to us", which we shorten to "is ours".
What's the difference in the usage of onze and ons?
I think you use "ons" with het nouns and "onze' with de nouns
Yes you use "onze" with de-nouns, and "ons" with het-nouns.
And know that we use "onze" with all plural nouns ( de and het )
Would one say, "Het is van ons" when offering a gift, to indicate the giver?