I'd say no, on account of Arcane Training only allowing you to use spell completion/trigger items as a first level character (in this case, since you don't have levels in whatever class you're using this on), but doesn't actually give you a caster level (which is what Magical Knack modifies).
That said, even without this combo working, you wouldn't need to make a UMD check; since you can activate scrolls as if you were a first level caster, it'd be a caster level check, with a DC equal to the caster level of the scroll plus one. That means to activate a scroll of a second level spell, that'd be d20+1 vs a DC of 4; a 90% of successfully activating the scroll. On a failure, you'd make a DC 5 Wisdom check to avoid a scroll mishap; barring a mishap, the scroll won't be expended.
