From version 10.2.x on, new versions of Threat Plates for WoW Classic will no longer be published here, but on the Threat Plates project page (as seperate version like for TBC Classic as well): https://www.curseforge.com/wow/addons/tidy-plates-threat-plates.
Threat Plates (Classic) supports Real Mob Health, an addon that records damage taken by a mob. This is used to estimate its max health when in combat and stores it for better accuracy in the future. Without it, all units will show with 100 HP health as WoW Classic does not provide accurate HP information via its API.
Q: Every time I try to increase the max distance up to which nameplates are shown, it reverts back down to 20..
In Classic, 20 is the hardcapped maximum value for max distance. There is no way to increase it beyond that.
Q: Quest icons are shown on all nameplates, not only on quest mobs.
In the Classic version of Threat Plates, the Quest widget is disabled (in the sourcecode) and the ! icon was not shown by ThreatPlates in all reported cases, but by some other addons like Questie, Azeroth Auto Pilot or ClassicCodex, as they added their icon to the Threat Plates frame. Version 1.1.4 fixes this issue.
Q: Durations off buffs and debuffs on nameplates are missing or not correct.
In WoW Classic, the game does not provide information about the (maximum and current) duration of buffs and debuffs. The library LibClassicDurations helps with that by providing an internal database of aura durations and monitoring the environment for newly applied auras. For long-running buffs and debuffs this does not work as they are applied outside of the range of LibClassicDurations so that their reminaing duration is unknown. For this reason, auras with a runtime with more than 30min are always shown without duration.