Supposably does mean "it is possible to suppose", as opposed to the very different meaning of "supposedly"!
Supposably is quite rare in actual use; I believe it's more an inkhorn term than one actually found in the wild, so to speak. Thus, I fear that when you encounter it you are probably really encountering somebody who meant to say "supposedly".
I do not know of any pair of words that have the same meaning and don't think it very likely (except certain alternate spellings and pronuncations of the same word, where even there it means something, except it is about you: your dialect and background, and there are often pragmatic factors at work). So this sort of question is always a good question, and often one that dictionaries don't help much with.
"OK" is particularly difficult, with many folk etymologies (like letterism for "Orl Korrect") and even an expansion or two (Okey Dokey and okay). It is very informal, and I believe quite universal across brands of English, but perhaps less in Britain. If you believe the Orl Korrect etymology, I think it relates to American Military jargon, presenting on parade. In the computer and mobile world, it has a special role as being short - so OK and NO are the positive/negative answers you can give in two characters. OK often means you are agreeing without really caring one way or the other.
"All right", or "alright", are different spellings of the same thing, and not quite so informal. You can also just say "right". In British English "right you are" is I think the origin of "righty oh" (or various alternative spellings thereof), and is clearly closely related. Alright often means you are agreeing against your better judgement, so is then pronounced in a grudging tone.
Of course both can be pronounced slowly or with strong falling tones to indicate negativity.
Both can be used not to mean "okay" but the opposite (that you have agreed with yourself to make a stand, make a move, or some such)
"Okay, that's enough..."
"Alright, that's enough..."
"Right, that's enough..."
But then there are some slang adjectival uses that tend to be more dialectically limited, and are thus harder to substitute (but people still do at times).
"She's a bit of alright!"
"He's an okay sort of bloke, I guess!"
Best Answer
Latter is at the end. Being the second of two persons or things mentioned; near or nearer to the end. It has more to do with placement, so to speak. Location, real or figurative.
Between captain and major, the latter is the higher rank.
My favorite is the latter part of the book.
Later has more to do with time.
She arrived an hour later than expected. Better plant your tomatoes later in the Spring than sooner; tomatoes don't do well in the cold.
They are similar, but not interchangeable. You can't say "I'd prefer the later" if asked if you would prefer coffee or tea. You can't say, "Plant them latter in the season."
You can say, in the latter part of Spring, as with the latter chapters of a book.
You can say, I'd like some coffee, but later, please.