The helicopter you pilot in "Last Stand" is identified as "car $5373", and, much later, after several "unknown_checks", becomes AP. Maybe it's AP as a wreck, because the immunities appear after "car $5373 wrecked". If so, it's outright impossible to get. Will check it when I get there.
So, I got to figure the Skimmer glitch out, and managed to enter the Mendez Mansion during free roam. I haven't checked if the duped Domestobot remains there after mission failure, but let's say it does*. Great. How do we get it out?
About UH helicopters (and boats to an extent). This is another strange case that needs more understanding. As mentioned a few days ago, when I obtained Reni's AP Maverick, it was awfully slow, more so than usual, and could barely tilt. Thankfully, after storing it in the garage, its handling reverted back to normal. I never bothered with the Armadillo from "Say Cheese", but I'm positive this is the same case. Perhaps the person who posted about this in GTA Wikia, obtained the helicopter with CheatDevice or something, but, since he/she couldn't store it, assumed the helicopter was UH. Which reminds me of the mysterious Jetmax, which, after storing in the garage, is completely f*cked up; half the boat is submerged underwater, handles even worse than before, and can't reverse.
Since I'm almost clueless on the matter, I made my own legend and definitions regarding UH. A Unique Handling vehicle to me is ANYTHING that handles differently from its regular counterpart. Doesn't matter if it's better or worse, as long as it's different. Since the Heavy property apparently affects handling, ALL Heavy vehicles are also UH to me. So, if I were to make my own guide, I wouldn't list every Heavy vehicle as UH, but would instead mention that the Heavy property increases grip in the legend. If anything Heavy behaves differently, such as the Jetmax, I would explicitly state it in its respective section. About the AP Maverick (and the Armadillo, if you'd like), I would explain its quirk, but would NOT list it as UH, since its handling reverts back to normal after storing.
So, if I effectively clear UH out of my dictionary, what about bikes? I would list bikes as IH, which stands for "Improved Handling", to match the original "Improve_handling" command used in the script, ALWAYS followed up by the bike's respective IH value. In my imaginary guide, I would explain in the legend that the standard IH value is "1.0", and ONLY anything OTHER than "1.0" had been added. Therefore, the VCS bike list would be:
H/AP/PP/WP/IH-2.0 Biker Angel
And the LCS bike list would be:
H/BP/FP/DP/WP/IH-1.2 PCJ-600 (needs more testing)
In my opinion, this taxonomy is more precise, and leaves little margin for questions, but if the community decides to stick with UH, I don't mind; I'll keep using this term on the forums.
Maybe the Jetmax is FP/EP during the shootout. No idea where the GTA Wikia staff base their research on.
AP/TP conversion? I'd say the best candidate is the Rhino. Here is why: H/DP > AP. In the case of the Firetruck; H/QD with double Health during "Karmageddon" > plain AP/TP. This conversion removes the Heavy property. The Cheetah is available among the handful of AP vehicles the game has to offer (what else could possibly be so much better than the Cheetah to be the chosen one?). The Rhino is so naturally heavy and destructive, that acts as if it had the property. It's also one of the easiest vehicles to convert because it's possible to start "Vigilante" with it. You can easily tip the Rhino back on its wheels if upside down.
*EDIT: It doesn't. I entered the mansion right after failing the mission, and it was not there. I once did something weird; I completed an order, and after the cutscene, the green robotic text disappeared, and I couldn't switch between functions. Also, Armando Sosa stopped giving commands, and I was permanently stuck inside the mansion controlling the Domestobot. Is that any progress? Nah.
EDIT 2: The Police car can't be converted to EC Black. The hitmen do enter it, but its colour doesn't change. I guess it's a livery in VCS, then? As I correctly suspected, likewise for the Polaris V8.
EDIT 3: I got the Biker Angel. Protection Racket worked. It wasn't easy, because Lance's health depletes very quickly, and the cops are annoying, but wasn't hard, either. **1/2, or *** max. The bike itself is phenomenal. Much better than I expected, and definitely superior to the PCJ-600 and the railshooter bikes in LCS. It's hands down the single best bike in ALL of GTA. No questions asked, no quarter given. It's FAST, its grip is astonishing, and it's WAY TOO DAMN HARD to fall off it. I don't know how the VCS engine works, but this thing is MUCH harder to fall off than the H/WP Sanchez, the railshooter Freeway and Angel in LCS, and ANY bike in San Andreas, even with maximum Bike Skill. You actually have to put effort to fall off this Biker Angel. For instance, I collided with a freaking Boxville head-on, at full speed, while leaning forward, AND THE BITCH RAMMED THE VAN OFF THE ROAD AND KEPT GOING! As if the Boxville was a goddamn paper bag. I was thinking it might not be as good as an AP NRG-500, due to worse bike mechanics than San Andreas (lack of Bike Skill), but it is. For those who don't mind OM0, this thing is a must.