Recap; because you're still missing the point.
Ensylum:
"Personally I feel like Joaquin Phoenix is going to be an amazing Joker. Almost as good as Heath Ledger’s. But what are your guys’ opinions?"
SfH:
"What makes Joker what he is, is Batman and since this movie won't have any of that, it's just going to be a decent character study. Not a proper DC Comics Joker film."
Ensylum:
"Joker and Batman are two sides of the same coin, and complete each other, if you set a coin down you only see one side of it. While having both completed the picture, you can still have a good Joker without Batman, he just wouldn’t be at his full potential."
Basically, you trip here. You've already conceded, via your own analogy, that a solo Joker film won't be at "full potential" without Batman. Hence, I chimed in with;
Etrigan:
"...a coin may have two sides, and even if you can only see one side, you have a very strong expectation of what is on the other. A solo Joker film without Batman is like turning that coin over and it is not what you expected to see. Not only are you confused and your expectations shattered, that coin is not going to be accepted as legal tender anywhere. It's counterfeit."
This was a broad analogy regarding the general cinema goer whose only concept of the Joker is films only. The only live action version on Joker to not interact with Batman is Jared Leto's incarnation. The only people who like that version are Harley Quinn cosplayers under the age of 21.
You then responded with this garbled concoction of all ideas discussed forced into one thought process;
Emsylum:
"You say that Heath Ledger gave a different interpretation, nothing more. You also say that people will have a strong expectation of what is on the other side of the coin. If, then, this Joker does as it has said it shall, and dive into the psyche of Joker, without Batman, then is it not just another different interpretation. This Joker incarnation is, oh, how did you put it again, oh yes, this Joker is “what is needed” for this film. If Batman was put into this Joker origin, it would therefore be completely and radically changed. So, rather than a failed attempt at making Joker without Batman, is it not a “different portrayal” of Joker?
The actor's interpretation, as you can see from above, is a tangent to the main discussion posed by SfH. I was addressing a different aspect of you OP regarding the subjectivity of favouritism before addressing SfH concerns regarding your coin analogy in a different paragraph. You have decided to combine the two and preemptively shout "Checkmate!" like a patzer.
I will make this clear. The solo film IS just another interpretation, however; that does not change anything regarding my stance on your question posed in the OP regarding the frivolous comparison of actors in the same role, nor does it justify your stance regarding the concerns of the film raised by SfH.
Now to summarise while addressing your final points. This solo film, as SfH pointed out, is an independent character study that just so happens to be using the Joker brand for marketability. It is most certainly an artistic interpretation of the character, not a representation of the established DC character. I am not saying that is a good or bad thing. I am not presuming Phoenix will be a good or bad portrayal. What I, and what I think SfH was saying is, the Joker comes with expectations. There are varying degrees of expectation as well. There are the simplistic expectations of the average cinema goer and there are the high expectations of comic readers, with various level in between.
The average Joe associates one thing with Joker; Batman. People, normal people, will sit down in this film having seen no trailers, no interviews, no media whatsoever. When they've run out of popcorn to occupy their brain, they will inevitably ask themselves — or more likely, out loud to the person beside them — "Where's Batman?" Some may even enquire, "Where's Spider-Man?" Such is life. The point is, the first hurdle this film has to jump is mainstream ignorance. This is not going to be a fast and furious action extravaganza a lot of people will be expecting it to be, despite the tone of the trailers. People are going to be confused, bored and angry when they get out, and poor old Phoenix's Joker is going to get the brunt of the feedback, hence; my views of comparing actors of the same role.
Then there are the real hard to please critics like SfH. SfH knows Joker's mania relies on his obsession with Batman and, to a degree, vice versa. Besides the absence of Batman, there are also long engrained rules regarding Joker in the comics which tend to cause debate when translated or messed with. Joker, as Nolan knew, is a more terrifying character if he is a man of mystery. Why has is taken so long for an origin story of the Joker? Probably because no true fan asked for one. Knowing everything about the Joker and why he is defeats the purpose of him in the first place. He is the great unknown fear. He is the personification of humanity's innate fear of what we can't explain.
The second key rule is that Joker can't die, nor can he ever succeed in killing Batman. In the context of film, the Joker should have no closure or resolution. This is not impossible, especially for a less mainstream foray to be open-ended, but a character study of a presumably good man turning bad is a risky punt when a film where the protagonist is a villain is already a huge gambit.
Finally, combining the first two rules, Joker should have no real motivation beyond his obsession with Batman. Everything Joker does is just an excuse to get Batman's attention. Needless to say, a solo Joker film devoid of a Caped Crusader is going to attempt to make the audience empathise with the poor victim who is driven mad by the world he's obliged to live in. Poppycock. No one should ever empathise with the comics Joker. The proverbial real world boogeyman who doesn't hide under the bed but knocks on your front door and shoots you in the spine.
I'm not saying Batman should be shoehorned into the film. I'm saying it goes against a lot of expectations across the board. And comparing Venom in Spider-Man 3 to Batman in Joker is completely inaccurate. That would be like Joker being shoehorned into a Batman film or, I don't know, Suicide Squad, perhaps?