I believe he does, but just want to confirm. Does Mauler Mithel's ability of "When attacking at Range 1, roll 1 additional attack die" add 1 attack die to a Snap Shot? I don't believe this counts as modifying per the text of Snap Shot.

Yes. Mithel's ability is worded to work on any attack (not just primary), and the rulebook says that rolling extra dice is not a modification.

Excellent, thanks!

You are correct... rule of thumb.... adding an extra dice to your pool to roll IS NOT modifying... adding a die with a result to your pool IS modifying.