And if their [sic] is an accident like described above, all their political enemies will rush in like hyenas, and exploit this mistake for their political goals: to discredit the Loremasters.
I think the phrase "an accident like described above" is a mischaracterization.
That is, such an incident would obviously not be taken as an accident but as a denial of that individual's right to rule.
Given that the crowns are the known [by all, especially the populace; i.e those being ruled over] accepted method of confirming legitimacy (which they are unless you are taking a different route in your game), if the 'drooling idiot' syndrome happened, then there would be quick, widespread acknowledgment that the [now] 'drooling idiot' was never a legitimate heir.
In the case of the sister's son's, it would be widely accepted that those (previously believed to be untrue) rumours about the princess' mother having cuckolded the recently passed king's father were obviously true (at least, now, in retrospect).
If the princess was a true child of the king, then her son would be a true heir.
Obviously, the son was not a true heir so therefore the princess could not have been a true heir and thus her mother must have had an affair.
Remember, especially in Jaiman, you are dealing with very powerful magics that are legendary in power and 'duration of existence' (i.e. these are from *before* that dark age which was millennia ago).
As for the fake crown, interesting idea (at least one of the original crowns was destroyed remember) but I think the loremasters would quickly put an end to that.
Or at least, they would try to retrieve the real one and have it show up at an inconvenient time so the populace would see two crowns and wonder "who's zoomin' who".
DonMoody