I can confirm the post from above about the time limit in encore mode. I forced the game to speed up by holding backspace with the DevMenu activated. The timer goes from 59"59'99 to 0"00'0. When doing this the "TIME" display flashes in red but there's no time over as I waited another 60 minutes and it looped to 0 again. What should be the rule about the time limit in encore mode ?
The Oil Ocean zone cutscene death glitch is also possible on several other levels. This video shows it for chemical plant zone 1 (1:26), Press Garden zone 1 (9:54), Oil Ocean zone 1 (18:18). It also shows a way to corrupt Oil ocean zone 1 just after.https://www.youtube.com/watch?v=y_TTbqjyyTo
I suggest applying the rule to all levels instead : "You can't die during a stage transition in such a way the next level will load with a frozen timer."
About the special stages, I think getting the emerald or not depends on the stat to submit. Stats like the time in both Sonic 4 games should require the emerald to be obtained to save your stat (else you just ram into goal blocks, red spheres, holes or such to loose on purpose as fast as possible). Sonic CD does that well, you need to destroy all the UFOs to get your time.
However stats like rings on Sonic 1 (genesis and GBA versions) and 4 shouldn't require the emerald since you can get the rings and they remain whatever you win or loose the stage.
About Sonic Mania I would say doing it the best way to get the highest score, getting the emerald or not as the score remains at the end. I think the emerald is better on some stages to save more rings for more points but you can also just fall in a hole (in other stages) after getting the last possible "BONUS!".
I don't know much about other Sonic games, some even lack a special stage chart (like S1GG).
Maybe we can get more opinions from other people. I'm sceptic about mania.