Coordinating Conjunction and Comma Question

[EmaNekaf]

I'll use the following sentence as a way to illustrate my question:

When Hank accused Mary of taking a cookie, it caught Mary off guard, seeing as she had never lied to him before and all twelve cookies were still on the tray.

In this sentence and others like it, I'm not sure if I would need a comma before the "and" or not. Technically, it's a coordinating conjunction joining two independent clauses. However, in my head, "all twelve cookies were still on the tray" is part of "seeing as" as another example of why the accusation caught Mary off guard, and putting the comma would separate it from that. Is it legal to leave out a comma if it is directly grouped with something earlier in a sentence as in this example, or am I just thinking about this way off, and I always need a comma before a coordinating conjunction when there are two joined independent clauses?

 

[jutfrank]

I share your thinking. I would leave out the comma for the reason you give. When deciding whether or not to use a comma, think about how the sentence reads more than about any hard and fast rules concerning coordinating clauses and such.