About the Beamshield thing - as far as I know from Gundam 'lore' if you could call it that, 'Beams' are the product of energised Mega-Particles from Mynovsky Physics (something they made up to make all this possible), the Beam Coating on sheilds are made of thesame particle lattice used to disrupt the Beam energy (they also deploy it in cloud format around the ships) and the Lohengrin is a beam of Anti-Electrons.

Taking those three things into account, the Beam Shield 'SHOULD' have provided no, lasting protection against the Antimatter Beam, aside form giving it another chunk of metal to go through (this is why the beam shields still melt when exposed to high amounts of heat and just dissipate beam weapons.)

I'm not sure what the Hyper Impulse Cannon acctually fires, it's supposed to be a Beam weapon, but it has the same animation as every other Antimatter weapon as well so... I dunno...

It makes sense, after a fashion, I certainly don't see it as a big mistake or anything, since Strike did not survive the explosion (but apparently Mwu did... FUCKING STUPID WIRTERS!)