I just argued your side, but I don't think this defense holds up.
Little white lies are objectively okay.
Little white lies are objectively okay.
They are subjectively ok. Some people believe lying is always bad.
I think the "true idea" (or let's call it "reasonable idea") when fully spelled out is "sometimes a white lie will be beneficial, will cause less harm, will inspire confidence in the receiver; and conversely, the harsh truth may harm them or be counterproductive". It's not the white lie itself that is the "good idea that is untrue"; the good idea is the reasoning behind the white lie, which is true.
Simply because someone believes something does not move the phenomenon to the subjective realm.
The Earth is not flat.
Some may argue a case for why they believe lying is always bad and would never engage in it. Yet experience proves those most vociferously against little white lies are often lying to themselves on a variety of topics. "Honey, does this make me look fat" is a no-win situation for these individuals.
From your POV yes. From their POV no.
I'm sorry, you cannot use this argument. It's not even in the same category.
Whether telling a white lie is ok is as subjective as it gets. You can pick better examples if you want to make the case that some ideas are objectively good or bad; this one is not it.