The potion is not forever
You need to drink some more after a certain amount of time. If it really did work forever we would have living dead wandering around still. Lisle is still unaware of this due to having only been a recent user. (Being only 71, she probably only drank it within the last 40 years or so.)
The potion is forever, but does not prevent injuries.
While the soul can operate the body forever, said body still needs to be maintained, and is still susceptible to traumatic injury. If the body undergoes clinical death, then it will eventually rot. Hence, how Helen and Madeleine end up. At the end of the film, their bodies have rotted to the point where they literally fall apart. If they had not suffered their fatal injuries, they would probably still be in one piece.
The small amount of the potion used on Ernest extended his lifespan
Since it rejuvenates his hand, it's probably that the effect eventually spread to the rest of his body. Since he was basically a broken down
alcoholic at this point, it's not likely that he would have lived all that long if he continued his life with Madeline. A small amount of the potion might simply slow the ageing process, as opposed to reversing it, since he has multiple children afterwards and a hugely active and much accomplished life after the age of fifty. It also explains how he got his aim back with his throwing arm and how he survived a plummet through the glass ceiling into the pool that should have killed him.
Helen's revenge plan wouldn't have happened had her therapist chosen her words better.
You see that as Helen's clinic therapist is giving her a harsh, but needed, talking to, Helen clings to her use of the word "eliminate" in accordance to Madeline. The therapist, of course, means she should forget about Mad and move on, but Hel takes it as "inspiration" to decide to kill/take Madeline out.
If the therapist had instead chosen better words, maybe even gone with "the real revenge comes with living a better life without them" or something then Helen would have maybe just kept with the part of her plan where she taunts Helen with her success and good looks. Maybe steal Ernest back through non-lethal means or something- or found a better beau altogether. One whom Mad wouldn't be able to steal away.
This would have worked all the better since Mad most likely would have ended up dead anyway, her fight with Ernest would have still happened, would have still been bitter, and who knows. She might have fallen down those stairs anyway. And then Helen would have the youth, the beauty, the fame, the love interest - and Mad is a walking corpse.
The small amount of the potion used on Ernest saved his life from the fall
When Ernest fell from the rooftop into the pool, a fall which should have badly hurt him, he survives with only minor injuries. It could be due to the small amount of the potion in him repairing him enough to avoid severe injuries.
Ernest never drank the potion, but he did use it
The potion heals and revitalizes when used as a salve, so he probably doused himself with it to revitalize himself so he could better enjoy his new life
- Ernest drops the potion straight down when he's hanging from his suspenders, and then falls himself, crashing through the roof of the pool and into the water. It's quite possible that the potion was sitting on the roof, not heavy enough to break the glass itself, and then went through when Ernest came crashing down on top of it, landing in the water along with him. Perhaps even being diluted by the pool water, the potion was powerful enough to grant him some longevity. We've seen that it can affect people through wounds, and he had some cuts after going through the glass.
Lisle wanted someone to mess up.
She gives her "warning" directly after Mad drank the potion, as opposed to warning beforehand, which would have made more sense, because Ernest was
right about being an immortal- it’s so
boring that some of the "dead" celebrities are making appearances to spook people- so she chose to give Mad and Hel the warning
after taking the potion because she knew they’d mess up somehow, also with the double duty of making them examples to the other bored Immortals as to why they have to stay hidden, in case they end up like Mad and Hel.
Lisle's group will be exposed sooner or later
For a group that's supposed to be selective and secretive, they're pretty terrible at selecting new members. Not only do they hand out the potion to two women with a history of animosity with each other, but they also don't even assign someone to watch over the new recruits and make sure they don't do anything stupid like killing each other or provoking someone into killing them, which would instantly blow the lid on the whole thing if someone calls the police.
