Sargability question - order of variables
The short answer is that the order you put these in makes no difference to SARGability. If you want this to be as efficient as possible, you can add a RECOMPILE hint, or use dynamic SQL to generate the appropriate WHERE clause.

Since you seem to understand the concepts involved, I won't beat any dead horses here, I'll just point you to a great source on the subject: Dynamic Search Conditions in T‑SQL