Every lie has as one of its purposes deceit - a lie is said in order to lead another to believe something that is false.
Circumstances of duress, such as lives being at risk (eg the often-cited “do you tell the Nazis that you have Jews hiding in your house” conundrum), in my view, may lessen the moral culpability of a liar, to the point of venial or even possibly no sin. I wouldn’t say that they necessarily mean that lying is ever obejctively other than sinful.
By your definition, the “mental reservation” examples given would also have to be seen as “lessening the moral culpability of a liar”. Seems a stretch to have to say someone committed the objective sin of lying but then had their moral culpability reduced to zero because it was for the other party’s good.
I can think of another example: if a terrible accident occurs, killing one family member and injuring the other severely, and the injured one wakes up in the hospital and becomes agitated about where is the other person, and you or the doctor say “don’t worry, she is just fine” in order to calm the injured person down and keep them from possibly becoming sicker, you have also lied, but the purpose is not to lead someone into error.
Third example: Father Michael Pro in Mexico went about all the time in disguise, essentially lying about his status as a priest, to keep from being arrested or killed. He was lying to many people all day long, only telling the truth about being a priest to those Catholics to whom he brought the Sacraments. He is today Blessed Michael Pro and just one step from canonization.
Fourth example: You buy a person a necessity, but you know they will not accept it if they know you paid for it, so you frame it as having come from some other source rather than telling them “here I bought you this”.
So when it comes to lying being a sin, I think the more correct answer is “depends on the intent” than trying to frame it as objectively always a sin but with 1001 exceptions. “Deceit” in this framework would seem to specifically mean leading your neighbor into an error that is or could be harmful to either you or them, not just any old error.