I am in? Возможно?

Возможно, но как тогда с продолжением?

I'm for, but he's against

Uly, "BE he's not" - так и должно быть?

🙏

but he’s not

IMO, this is weaker than "против" because tнpically there are three options: за, против, не знаю

IMO “he isn’t” expresses the opposite position. Otherwise I would’ve said “I’m for it, but he’s not sure/he doesn’t know”

Otherwise I would’ve said “I’m for it, but he’s not sure/he doesn’t know”

Again, this is one option out of three, i.e. stronger than "isn't (for/in).

he isn't = he's against OR he's not sure (i.e. includes two options)

But here there’s a polarity because there’s only me for it, and the negative, which would be him against it.

OK, let's agree to disagree. IMO, there can be different degree of polarity.
There is a difference between "he's (strongly) against" and "he isn't for".
I added "strongly" only to highlight the difference.

Adding STRONGLY to one side would throw off the polarity. Then you’d have to say “I’m for it, but he’s strongly opposed.” Otherwise, all things being equal, the opposition in my translations is balanced, and BUT activates the opposite polarity at play in the second part. Also, if there were three elements, this polarity wouldn’t work: “I’m for it, but Grumbler’s against it and so it Tatiana.”

Sorry, but I don't understand your logic and can only repeat:
он против - this excludes "he's undecided"
he isn't for - this doesn't exclude "he's undecided"

"Strongly" just increases the degree, of objection/disagreement/...

he’s strongly opposed = он категорически против
he’s opposed = он против
he's not = он не "за", т.е. он против или не уверен

What you’re not seeing is that because there are two elements here - him and me, there’s a polarity, which perhaps doesn’t exist in Russian. In that by using a negative in one, then BUT, then a positive, or vice-versa, the opposite position is expressed. “I eat fish, but he doesn’t (eat fish).” “I believe in God, but he doesn’t” “I’m for Trump, but he isn’t” In all these examples the opposite holds true by virtue of the polarity presented: he doesn’t eat fish, he doesn’t believe in God and he’s not for Trump = he’s opposed to Trump = he’s against Trump.

The polarity does exist in Russian.
Однако, по русски тоже можно было бы сказать, "Я за, а он - нет" - и это было бы близко к твоим переводам.
Однако, сказали более сильно "он - против" (т.е, он именно против - не за и не сомневается).
Не понимаю, почему я не могу тебе это объяснить.
Сдаюсь.