So for Fou, in FGO he's a potential Beast IV that is defeated by accepting the good parts of humanity and making a sacrifice of his sapience to preserve Mash. It's less that he's hiding and more that he's observing. If humanity failed its trial in canon or here? Advent Beast IV, Cath Palug/Primate Murder.
I don't know about the magecraft being stronger. I'd say that they're no stronger or weaker than normal because incinerating human history has effects on everything else. Just because one thing fuels the fire more doesn't mean nothing else is consumed.
Matthew was initially grown in a lab like the rest of the Demi-Servant experiments. Even with Morgan's modifications he still has human influences, so is subject to both Alayashiki and Gaia. One of these is under direct attack, and has Opinions about humans connecting to the Origin. Using the Universal/Original/Unified Language involves connecting to the Origin, which left an opening.
Now, thanks to Morgan, he is also available to be tapped by Gaia due to his fae heritage. ...it would be a little awkward, to say the least.