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Barring that, the first may be Beast Busters: The Second Nightmare
. It has the bikers and tentacle school bus, atleast.
Yes, that is most certainly the game I've been looking for! All of the bits I mentioned were there, save for the visual style. It looks more akin to House of the Dead than Area 51, but I can attribute that to my shaky memory. Infinite thanks, my friend! The door is still open for suggestions on the second game, however.
For the second game, you're suggesting that the boss itself might have been called "The Tower"? I thought that might have pointed to a House of the Dead game because of its tarot-themed bosses, but the one instance I can find of a boss referencing The Tower
(from the second game) doesn't match your descriptions, nor do the other bosses from that game. I also don't know if any House of the Dead arcade titles used light guns bulkier than handguns.
Thanks for the suggestion, but I'm one-hundred percent sure that it isn't a House of the Dead game. If it helps, the game I'm searching for is similar in concept and visual appearance to Invasion: The Abductors, mentioned above by Glowsquid. However, there is a major information shortage on that game, and the handful of videos I found didn't include the details I described. In addition, all of the cabinet shots showed that it used handguns and not the larger guns I remember. Be wary that there is a fair chance of the game being two games that I've combined into one. Thanks for your help, regardless.
Teraburst.
TERABURST!
Over a year has passed since my last post, and I can triumphantly state that the second game I was seeking is Teraburst
.

I have two (possibly three, as I'll explain below) arcade games that I have been struggling to remember for years now. All of my searches on the web over that time have turned up nothing, not even the smallest breadcrumb. This is my last resort, and I don't have much faith that anyone will find what I'm looking for. I apologize in advance for my vague information.