Is there anything, a condition or a situation, that can make a dream about seeing the prophet untrue or just an imagination or a thought from shaytan?
The companions of the Prophet Muhammad (PBUH) knew him by face, so if they see him then its no doubt that the dream is NOT from the Devil. As we do not know him by face, we can not be sure that it is indeed him that we saw. So it's better not to make any judgement on such dream, but rely on Allah's and our Prophet's guidance and our God-given intellect.
Prophet (صلى الله عليه وآله وسلم) has said that it is impossible for shaytan to take His form in someone's dream. That is authentic Hadith. Do not doubt the sayings of Prophet (صلى الله عليه وآله وسلم). If someone doubts his sayings, he becomes disbeliever and whoever tells a lie about Prophet (صلى الله عليه وآله وسلم) he will be punished here and in the hereafter.
Now to your question: What are the conditions that violate vision of Prophet (صلى الله عليه وآله وسلم) in dream?
One is lying; a liar is forbidden from seeing Him (صلى الله عليه وآله وسلم). But every thing happens or does not happens is by ALLAH's Will. (citation needed)
A person who sends salutations (darood sharif) upon the Prophet (صلى الله عليه وآله وسلم) with deep love will continue to get closer to the Prophet (صلى الله عليه وآله وسلم) and will eventually see Him in his dreams. In the dream, they will KNOW that it is Prophet (صلى الله عليه وآله وسلم) as it is said: Momin's spirit knows other momin's spirit. (citation needed)
This is a hint to understand what happens in the spiritual life.
I ask forgiveness if there is any mistake from ALLAH AL-REHMAN.