You know, a lot of your symptoms are dead-on consistent with eosinophilic meningitis, which can be caused by the rat lungworm parasite, also known as Angiostrongylus cantonensis:
The parasite does exist in southeast asia. Snails and slugs are an intermediate host for the larvae. The snails/slugs crawl across lettuce, etc. that eventually get eaten by humans. The worms make their way to the central nervous system and brain, causing meningitis — symptoms of severe headache, stiff neck, nausea, and more. Sound familiar?
Did any of the infectious disease doctors test your cerebrospinal fluid?