1. You'll have noticed that there's a range of reactions to my "manage to [X]" example. Some of you agree with me; for some of you, the details are a bit different; for some of you, there are exceptions to the negative quality of the item; for some of you, the item can actually be positive rather than negative. All of which, it seems to me, is support for my suggestion that it's wise to be very careful when you say or write or sign "manage to [X]." You know in advance that it's an item that some people will find insulting, and you can't know in advance whether the person you're talking/writing/signing to is part of that dialect group or not; therefore, it's wiser not to use "manage to [X]" if you can find some other way to word what you want to say/write/sign.
2. The term "presupposition" has a variety of accepted meanings, depending on the field in which it's being used and the individual who's using it. When I use it in this LJ, this is my definition:
A presupposition is something that a native speaker of the language knows is part of the meaning of a sequence of that language even when it's not overtly present anywhere in the sequence.
3. You can't know whether something that is presupposed is true unless you have some external way to verify or disprove it; what you know is that unless the person speaking/writing/signing is deliberately lying, that person believes that it is true. So .... When someone tells you that John has stopped smoking, you don't know whether it's true that John at some point started smoking, but you do know that the speaker/writer/signer is (a) deliberately lying to you, or (b) believes that John at some point started smoking.