The effect of “Stumbling” goes on a chain immediately after a monster is Summoned. As a mandatory Trigger Effect, “Stumbling” is always on the chain before optional Trigger Effects (“Mobius the Frost Monarch”) or chained Spell & Trap Cards (“Trap Hole”).
“Stumbling” changes a monster to face-up Defense Position, not face-down Defense Position.
When “Stumbling” and “Final Attack Orders” are both active, a Summoned monster is changed to Defense Position, then changed to Attack Position.
If “Stumbling” is in effect and you Summon “Exiled Force”, and your opponent has “Trap Hole” Set, “Stumbling” initiates a chain with “Stumbling”’s effect as Step 1. Your opponent can activate “Trap Hole” as Step 2. You cannot chain “Exiled Force” because its effect is Spell Speed 1. Therefore “Trap Hole” will destroy “Exiled Force” before you have a chance to Tribute it.
If “Dream Clown” is Summoned while “Stumbling” is active, and changed to Defense Position by “Stumbling”’s effect, “Dream Clown”’s effect activates after “Stumbling”’s effect resolves.