The king won’t throw you in prison, other people will.
The Lesse Majeste law in Thailand states that anyone can bring the charges against anyone else for insulting the monarchy. It also compels the police to investigate every complaint of Lesse Majeste.
The king of Thailand does not make the prosecutions himself (well the last king did not - this new one might)
The (Perhaps apocryphal) fairy story told to newly arrived expats tells of two drunk Germans in the back of a taxi making fun of the numerous images of the king. The taxi driver said nothing, but drove to a police station and filed a complaint. It cost the Germans a great deal of money and a little liberty to get away from it.
Mostly in Thailand the LM law is used by political opponents to mess with a rival.
In England the only person who can bring a LM charge is I understand, the queen herself.
As to the allegation that Asian countries will readily execute foreigners on the discovery of something that looks like a drug, is one that underlines why we must get our news from multiple sources and dig the back ground too.
Malicious prosecutions are almost always financially or politically motivated. So on the rare times a drug trafficker is prosecuted it’s financial (didn’t pay of the police) or political - like the extrajudicial killings in Philippines in its so called war on drugs.
The verdict today from the Constitutional court regarding Thanathorn and his disqualification as an MP has a significant impact on the intermediate and long term future of Thai politics. Prayut and Prawit may be severely detrimental to the advancement of the country but the other sabre rattling General in the background would be a disaster. Another Coup d'etat? Based on that, the growing xenophobia and the strong Baht I would not consider retirement in Thailand.
None of this should stop a retiring person from considering Thailand - though the cost of living there is much higher that 10 years ago.