Would Weakness Gaurd eliminate (for 1 turn) both weaknesses on the new EX Pokemon (the ones that break 120HP rule). I assume it would, but we know what happens when you assume..........
I would say yes because on an EX Evolution card it says Weakness: and lists 2. It doesn't say Weaknesses, therefore the card would refer to both of the EX Pokemon's Weakness.
I can also see the old "Magnifier" being useful against certain Pokémon ex. Imagine facing a Pokemon ex who was more powerful than, and resistant to, your biggest Pokemon in play.