Repeated thanks to those who submitted clues for this.
My opinions are below. The winner is revealed at the end.
Big mouth from Mississippi, maybe, changing regarding recent killer? (5, 7)
Big mouth, etc. = DELTA; changing = VARIANT; def = recent killer
-- The two indicators for DELTA and VARIANT aren't that cryptic. The surface reading, I presume, alludes to vociferous doubters re the virus, its guises and its avoidance, but it isn't terribly elegant.
New problem emerging when interval data reclassified (5, 7)
INTERVALDATA anag; def = new problem
-- A very good find with 'interval data', which is/are a real thing, I find. The definition, while relevant now, is a bit vague generally.
Dangerous but ignored threat attired, naval-style (5, 7)
ATTIREDNAVAL anag; def = dangerous but ignored threat
-- Not a huge fan of the way the anagram is indicated. Not quite sure what to make of the surface reading. The definition, as with the previous clue, is a bit vague.
To many ducking the jab, a death blow is dealt? (5,7)
DEALT = DELTA 'VARIANT'; def = to many ducking the jab, a death blow
-- I'm not the hugest fan of using an implied anagram indicator as part of the definition, but I have seen it in many another clue. The reference to 'jab' gives the definition some specificity (cf Duke's previous two clues).
". . . or NOT to be" belated confusion everywhere it seems to this dreadful development (5,7)
BELATED anag minus BE = DELTA, with 'confusion' serving as anag indicator as well as pointer to VARIANT(?); as per David: "everywhere" alludes to Hamlet's "sea of troubles" and Coleridge's "water, water everywhere" seemingly!
-- I'm not sure how the allusions serve the clue or help the solver.
My winner is Mark. Duke and his interval data is/are second.