Well the thing is, human civilizations in general are made up of groups, and groups try to exclude outsiders from it on principle by way of their language, as it makes whatever they're on about, "theirs". It's the same with fan nicknames and in-jokes. That and to be honest, "Hyperbolic Time Chamber" and "Room of Space and Time" take a bit too long to type than their abbreviated counterparts, and if anyone is in a discussion using those terms but doesn't know what either of those terms mean or that they're interchangeable,
I'd be hard pressed to call them a fan. It's assumed someone wanting to engage in a topic about Cell
has seen the Cell Saga and therefore knows that the characters went into the "weird white gravity room" at some point to train-- either they know what the name of it is, or heard it but forgot. Then whether they use HBT or RoSaT is personal preference. It's like going into a DMC discussion and not knowing what "DT" or "DDT" is.
II. Okay I worked it out. The "Cell Timeline" where Trunks died in is one where Cell ultimately didn't make an appearance in the past at all. In that timeline, Trunks went back to help with the Androids, he and Krillin ventured to Dr. Gero's lab and found the blueprints for the shutdown remote, but they did not go underground to where the Cell Project was happening. They shut down the Androids, and no (born) Cell meant no Cell Games. That was theoretically the best timeline for the Z-Fighters since no one died,
but no one trained as hard, Gohan didn't turn SSj2, the Androids are never reformed so Krillin and 18 never hooked up, 17 didn't become a park ranger, etc. The Buu Saga might still have happened but that'd be speculation. Anyway, Trunks would take the shutdown remote back to his time, defeat
his androids with the same method, but by that point Cell was born. He didn't find the Androids since they were shut down, so he killed Trunks and hijacked the machine.
Cell's hijacking sorta retconned this unseen timeline and he ensured his own death. He showed up, and Trunks (on his second trip) eventually found the Time Machine that was left abandoned, that wouldn't have been there before. Cell fought the Z Fighters, then blabbed about the underground lab and the Cell Project and how he got there. The result? Trunks and Krillin not only found the blueprints, but they went underground and destroyed the lab, killing Cell before he was even born (lol aborted). Future Cell's continued presence in the Series Timeline meant that Trunks
didn't go back home so soon to defeat the Androids. Cell became Perfect, started the Cell Games, everyone trained their asses off, Gohan went SSj2, Goku died, Cell killed Trunks out of spite then he got vaporized by Gohan. But! Trunks is brought back to life and the Androids are reformed, so Trunks then leaves from here to his future, where he kills the Androids by brute force and wipes Cell out before Cell can kill him and go back in time. So basically Cell killed Trunks twice and himself twice through his own actions. I can't seem to figure out why Trunks would need to go back to the past the second time around, but that's now a minor issue.
III. That reminds me that there's a minor plothole, in that Vegetto separated almost instantly after being absorbed despite Potara being a permafusion, but Buu absorbed Gotenks at SSj3 and got the boost for a full 30 minutes even though SSj3 reduces the amount of time to 5 because it's so energy intensive. But skipping that, yeah Bulchi would need to be absorbed by Buu to separate. Then we'd have to hope that Bulma and Chichi know their way out unless Mr. Buu expels them himself.
IV. He
wouldn't do it, even if he
could do it. He'd likely object to using a form or technique he didn't straight-up train his ass off for, and using Goku's moves would probs make him sick. That's why he's never used a Kamehameha before. It's a pride issue, lol. Anyway, slap Bulma in front of him and he wouldn't need SSj3. [/shot]
V. Hmmmm... Super Saiyan 3 was definitely not a natural transformation, since Goku pulled that form out when he was already dead, and the dead don't have the stamina limitations of the living as their "bodies" are ageless. Goku said so himself:
"Super Saiyan 3 is only supposed to be used in the afterlife. It uses too much energy for this world of time. You get instantly worn out!" So it drains stamina/energy far quicker than normal in order to maximize Ki output, whereas other SSj transformations are more evenly distributed.
Where you say "SSj3 would be better if it looked unique in appearance to each character", I'd say that's the purpose of SSj4 in GT. It drains stamina at a much slower rate than the SSj3 transformation does, looks different to each character, and seemed to be taking advantage of the
natural Saiyan characteristic of having tails and transforming into their Great Ape forms (where the Great Ape form itself doesn't drain stamina, it stays until the full moon is destroyed, leaves, or their tails are cut). SSj3 isn't natural so that's why it all looks the same no matter who does it.