I see this exercize as about coming up with a range of potential explanations/outcomes of the bright spots on Ceres, not about deciding which one is correct. That's a loser at the moment. So I'll just concentrate on developing my covert human base scenario; I don't need to disprove the alien track. Nor could I--there's nothing wrong with your arguments.

I have to flat-out disagree about the impossibility of getting humans to Ceres sometime after the second half of the 20th. The point I think you've forgotten is that feasibility is judged against criteria like safety and the fear of public failure. By NASA's extraordinarily risk-averse standards, damn near everything is impossible and needs further study. Even LEO is still experimental territory.

But if you're sufficiently motivated, and operating in secret, you might accept lousy odds using primitive tech.

It's not a question of figuring out how to build a rocket way back in 1945--they'd mastered that--it was a matter of building a rocket big enough to launch something past escape velocity (and once you do that, you're already more than halfway to anywhere). It took von Braun two decades to get from V2 to Saturn V...and what if von Braun represented Germany's B Team? So the basic problem is just brute force rocket power, and if you're not worried about efficiency or elegance or cost, you just build a damn big V2. Or, by the early 60s, you build an Orion monster.

Life support? Seems to me that the problem becomes more severe the smaller the spacecraft. See the preceding paragraph...use a monster booster to launch a monster habitat including a greenhouse big enough to hedge your bets, and big tanks of water and air, and a small nuclear reactor. Sounds pretty cozy, actually. Set the thing down on Ceres with its nice feather-light gravity, and there's your base, prebuilt.

And can you launch the thing without being caught? You're thinking in modern terms, but surveillance of near space has evolved from zero after WWII. I'd say up until around 1960, the US or USSR could have launched a deep space mission unobserved.

But afterward, especially after BMEWS and DEWS radars (and Soviet counterparts) kept watch on the poles, think about how many false alarms there have been. Several times both systems have detected what they thought were over-the-pole ICBM launches, only to have the blips mysteriously vanish. So maybe the system worked, and maybe the "blips" were headed north above the plane of the ecliptic to exit the Earth's vicinity in a direction that's generally not being watched? (In simpler words, more people at the equator than the poles to watch the skies.)

(Rereading the last few paragraphs, I feel compelled to disclaim that this is just an intellectual game. I'm not actually advocating that any of this is real, despite using assertive language.)

Sure, this is ridiculously speculative and outlandish and all that, but impossible? No. I hate that word, find it bitter upon my tongue, and I try never to utter it unless forced by hard reality.