Who was smoking that cigarette holder in the last panel?

Mark - my guess would be the Impossible Man. I think he picked up that habit from the Thing.

Hawkeye will later play on Tigra's interest in the Thing to keep him around in the early issues of WCA (so that might count as an inbound reference). I always wondered at that time how on earth Tigra ended up hanging with the FF.

This issue and the next few are a sign of the troubles at Marvel, as it was very clearly cobbled together at the last minute. The robot in this story, fior example, is never really explained; the Marvel Index finally has to decide that it's probably a dropped thread of the rather slapped-together Mad Thinker stuff that turns up as part of the resolution of the Brute plotline.

What a cliffhanger. I was so worried for Reed. I wasn't able to see the conclusion until several years later.

I can't recall; has there ever been an on-panel explanation as to how, while in open space in the Negative Zone, people are able to breathe, survive radiation, avoid decompression and generally endure all of the other things that would kill a human instantly if they were exposed to space without protection in our universe?

I can't think of a time it was ever addressed, nor why so many floating boulders have 1G gravity. The negative zone is like the Ditko-verse that way.

From the wiki entry on the Negative Zone -
https://en.wikipedia.org/wiki/Negative_Zone
"Curiously, for as little life as there exists in the Negative Zone, it is very capable of supporting life. There, outer space itself is permeated with an oxygen-rich atmosphere, closely approximating Earth's. Consequently, humans can exist peacefully in space without the need for cumbersome pressure suits or oxygen masks."

This also may answer the boulder issue -
"It is perhaps an issue of gravitational pull that is one of the biggest hindrances to life in the Negative Zone. While all objects of reasonably sized mass (planets, moons, asteroids, etc.) obviously have their own gravitational pull, it is weak enough to be overcome with minimal effort. Most heroes with flight capabilities can escape a planet's gravitational field with ease, as can any machine with the capacity for flight."

Aah, comic-book science. There's nothing like it.

Regarding Sub-space/the Negative Zone: In FF #51, the Thing impersonator described conditions there as "extreme" and said that only someone with a protective helmet or the physiology of the Thing could survive. Then in FF #61, whoops, Reed gets sucked into the zone without a helmet, and survives. I think this probably happened only because Kirby was always working without any reference to previous issues. The whole pseudo-scientific explanation for sub-space was actually pretty consistent, internally to #51, but then there's a big continuity slip at #61, and everything about the zone after that has been haphazardly and repeatedly retconned, in various efforts to make it the stories reasonably consistent with each other-- which is more or less long past impossible at this point.

To me it reads like everything from #48, "The Coming of Galactus," up to #60, which finishes both the Silver-Surfer/Doctor Doom storyline and the Inhumans sub-plot storyline, was planned and plotted in one go, and then continuity slips, not just in this regard but also in a number of other areas, for whatever reasons.

I'm actually disappointed with the explanations for the Negative Zone! I always thought it'd be something high concept like it being a different reality with it's own physical laws which only seem to resemble ours but operate in completely alien ways, like the whole place being governed by perception or somesuch...

"There's oxygen everywhere" and "gravity is a bit weaker" just seem like they aren't even trying. :(