There was no mention in your second last post that Corban was preparing for a riposte, so I must assume his actions are all post-haste. There was mention that he'd already shoot out Ishtalle, though, and 'immediately' after.
Split seconds was a reference to everything that would be going on in that moment, and how long it took Myron to lock in on the burrowing Corban. It is mentioned specifically that there is less than a second between the sheath striking his shield, and the laser beam being set off, this is the timeframe Corban has to fully create aerogel, enchant it, and enchant Ishtalle to deflect magic. (Also, wasn't his enchanting ability nerfed?)
If you're going to use a real-life synthetic material as an example, it should follow the laws of its own physicality. Aerogel is one of the lightest materials on earth, there is no doubt that the beam would thrust Ishtalle right back in to Corban. After some research, I've also learned that aerogel & areographene have a melting point of about 2,250F, which means it cannot insulate against lava. A disintegration ray has the heat and pressure necessary to not only melt rock in to lava, but to eradicate it entirely. Both melting and shattering are likely scenarios for the aerogel, even if it is enchanted beyond the norm. We're talking fifty times hotter than aerogel can handle. (EDIT: More research, carbon aerogel melts at about 6,422F, which still doesn't protect it.)
You weren't aware that the beam had the force of extreme gravity behind it, so you didn't post accordingly to what would happen if it did. This means edits need to be made. Beacon of Nihil is stated to be the combination of two major runes, gravity, and erosion, that is because it has the effects of both. It is not a hyperpowered flashlight, it is one of Myron's most powerful spells; and he only has a few purely offensive spells.