How, if at all, could the case of Etienne Rarnis provide a casus belli for the Empire?

I dont know. I Just kill him when I see him.

It couldn't. The Empire isn't going to declare war on the Dominion just because some Breton thief nobody got imprisoned by the Thalmor.

The only reason I know of that name is because I had killed him recently to test his corpse for permanency; I sincerely doubt anyone cares about this fellow, let alone enough to start a war over him.

The question of the OP does not require caring about Etienne Rarnis per se. If the Empire was looking for a casus belli and wanted to use the case of Etienne Rarnis to that end, then could it provide a plausible casus belli? If so, then how?

He's acting outside of the law and I don't think that the Empire would really care about him unless they desperately wanted to start a war with the Dominion. But technically the Empire could not gain casus belli because depending on the progression of the civil war Riften would be under the control of the Stormcloaks, out of Empire jurisdiction and therefore not one of their citizens, so no right to declare war would be gained.

How do you understand the OP to refer to the "case of Etienne Rarnis?" As a Breton, he is a citizen of the Empire by birthright.

Closed at the request of the OP.