Another fun discovery today. "Invincibility Mode" is actually not invincibility at all. It's just a whole bunch of extra hidden bottles on the meter.
What I've been referring to as "Invincibility Mode #1", which is described as being able to survive 1 fatal hit without dying, is actually 160 extra bottles (off screen), which function exactly the same as normal bottles. Including the 15 bottles meter on screen, this gives a total health of 175 normal hits. IE, after you stand around and let a guard hit you 160 times, that 161st hit finally makes a change on screen, turning the 15th bottle dark, and leaving you with 14 more hits.
Similarly, "Invincibility Mode #2" is actually 240 extra bottles off screen, for a total health of 255 normal hits. IE, if a guard hits you 240 times, or you fall down a 2 level gap 240 times, the next time you do it will make a difference to the on screen bottles.
The important discovery is this: fatal hits don't have a special parameter that tells the game whether or not to kill you. They simply do more damage. 100 hits! So when you're in INV#1 with your 175 health, the first "fatal" hit does 100 damage, and leaves you with 75 health, not enough to survive another 100 damage. Whereas, if you have 240 hits, you can survive 2 separate "fatal" hits, and still have about 55 total health to work with. I have shown some setups where you can knock an overhanging loose panel, land on spikes, and then the falling panel hits you and returns you to a playable position (instead of being stuck on the spikes). In a case like this, you take 100 damage from the spikes, and an additional 1 damage from the falling panel, for a total of 101. If you do this process twice with INV#2, you have survived in a playable state, taken 202 damage, and have 53 remaining health, and can take another 38 normal hits from a guard without noticing any loss of health on the original 15 on screen bottles.
The Crusher, the Chomps, the Spikes, the Sparkles, and being hit by a guard when your sword is put away, all do 100 damage. And falling from a height of 3 levels or more is also 100 damage. Fire Walls on Level 11 do 100 damage. Being stomped by the Monster on level 17 without your sword drawn is 100 damage.
One possible exception to this rule, which cannot readily be tested, is Jaffar's attack when he lifts you in the air, and destroys you with a fireball. When I very quickly access Debug Menu, I can select "Boost Meter" to restore all of my health, and stay alive for another hit. But eventually, the hits overtake too quickly for my health reset attempts, and I die. I think if Challenger was able to run a test tas, you could enable debug menu, jump to Level 8, do the quick warp glitch off the ledge after the first guard to acquire Invincibility Mode #1 (160 invisible bottles), then jump to level 20, play jaffar, and let him do his air fireball killer move, and see if you can access Boost Meter on every frame, to see how long the TAS can stay alive. lol!
I made some fun illustrations for the meters, below:
INV#1 (160 extra bottles, 175 total health points):
INV#2 (240 extra bottles, 255 total health points):
..
I've also confirmed that in both cases (+160 and +240), large bottles, debug health boost, and merging with ghost will only refill existing meter to full status. These do not create extra meter count. Presumably, these are only programmed to increase meter count when there are less than 15 bottles onscreen.
I can also report that in the past, I have done glitches where the Prince survives a fatal hit, and loses onscreen health meter. Previously, I thought it was some sort of broken setup, but now we can use new knowledge to suppose there is a setup which gives something like +95 invisible bottles, so there is enough total meter to survive the 100 hit points, but not enough to conceal the effects on the visible meter.
Now, feast your eyes upon this insanity!!!
I may create a series of videos for this, even though it's just a silly Game Genie code, because the results are so crazy. I daresay, if ever Challenger was to consider making a Game Genie TAS, this would be the one!
I left some of them blank, as there was essentially no potential for improvement. I just happened to complete levels 8 and 15 here, although they also seem to have no major advantage. I think perhaps level 8 could have a 1-2 second improvement if I take the time to polish it up. However, 1-2 seconds is small potatoes compared to the other devastating time saves. Lol.