Is it a splice comma if an interjection-phrase is involved?

It's certainly poor style, if not actually wrong, to join independent clauses with a comma when a semicolon or other punctuation would have sufficed. But interjections are usually offset from other clauses by commas, as in:

Dang, what do we do now?

It seems to me that this naturally applies to certain longer phrases, such as:

I don't know, what do we do?

In which "I don't know" functions as a single interjectory unit. According to usual style, that comma ought to be replaced, but to replace it subtly changes the pacing of the sentence, doesn't it? So, does a comma used in this fashion count as a splice or not?

Valid examples:

• I'm sorry, who are you?
• What the heck, you only live once.
• You're welcome, it was my pleasure.

Invalid examples:

• I'm sorry about that, what can I do?
• What the heck is going on, these zombies are ruining my earlier example.
• You are of course very welcome, I'd be happy to lend you my shotgun.

The basic statement that independent sentences are not to be connected by commas is disputed. Fowler, of all stylists, calls it superstition; he mentions several factors that may or may not be enough to warrant comma splices: short sentences, a strong link in thought, and the use of certain conjunctions that allow comma linking more than others. Perhaps there were other factors too.

Depending on the sentence, you could separate it into two sentences or use a colon, semi-colon, or (my favorite) a dash.

• I'm sorry -- who are you?
• What the heck? You only live once.
• You're welcome; it was my pleasure.

I think that technically speaking, these are indeed comma splices:

I'm sorry, who are you?

You're welcome, it was my pleasure.

This, however, is not:

What the heck, you only live once.

(Because "what the heck" is not a complete sentence.)

That said, I don't think using a comma with any of these is incorrect. Trying to punctuate them with something other than a comma would inevitably change their meaning, which would be way more incorrect than just living with a slight run-on sentence.