Can't believe they tricked me into watching this.
There was no trick. The fact that this was just a prelude with the tie-in only being in the final minutes was overtly stated in the advance publicity and interviews. Okay, the promos on TV were a bit misleading, but that's only to be expected of advertising, which is why it's best not to rely on it as an exclusive source of information. And really, it stands to reason.
Supergirl is set in a different universe from the one where Earth is being invaded, so how could it be more than a peripheral part of the crossover?
And that kind of setup has precedent in comics, due to their serial nature. I'm reminded of the way Marvel's original
Secret Wars miniseries was set up by the various comics whose casts it brought together -- the preceding issues just had the characters get swept away in the last page or two, followed by a "To Be Continued in" notice like we got here. So it did feel very comic-booky.
Actually, they managed to tie it in more than I expected, with Vibe's portals showing up twice in the course of the episode and even helping to save Supergirl from Henshaw. That was a clever touch.
What wasn't so clever was having James, Winn, and Alex have a whispered conversation about the Guardian while Kara was just a few feet away. Umm, super-hearing, anyone? I know it's a pretty common convention in screen adaptations to assume that Kryptonians' enhanced senses are only engaged when they choose to turn them on, but I tend to assume it's more a matter of what they choose to focus their attention on at a given moment. It's hard to believe Kara didn't overhear them.
Also, they explained why the Medusa virus didn't hurt Kryptonians, but they neglected to explain why it didn't hurt humans.
I hadn't expected them to wrap up Cadmus (or at least Lillian's part in it) so quickly -- or to cure J'onn so quickly. That was a bit anticlimactic. Still, I saw what Lena was doing a mile away, though I guess they did a pretty good job of making it credible that she might actually be helping her mother rather than sabotaging her.