No. That won't work, because Noivern is faster than Gardevoir and her Mega-evolution, and Skill Swap has no increased priority. Boomburst will still be Normal type on the first turn.
I'm not even sure Gardevoir will Skill Swap Pixelate, considering that on the turn you mega evolve, your Pokemon will still have the stats and I think ability of its normal form.
This takes way too many moves for it to happen, and Noivern doesn't even get the benefit of STAB from Pixelate. Fairy is x2 against Dragon, Fighting & Dark, and 2 of those Noivern already covers with its STAB. What would be the point?
As for Mat Block, it also has no increased priority, so something can still go before Greninja and use a move against Noivern/ Greninja.