Hey. You make so many excellent points, thank you so much for the searching and thinking and writing !
I would say having clothes from Portugal and Spain was a really common thing here in Switzerland back in the 80's and 90's. A lot of fabric and clothes were imported. I had jeans and shirts from Portugal and Spain. I grew up in Switzerland in the 80's.
The spy lead, which I was a little reluctant to consider, is not off the table. Lebanese Christian ? possible. Considering this was the only wealthy enough community to move to Europe during the war (my mom was a lebanese christian). Also, he could have been a swiss spy, we have secret services that are shady af too.
I find it super odd to have found litterally no article mentionning the case, in european press or american one. I have been cruising through archives and all. Not enough closely for now I guess.
About political activism, it is possible too. As you said : red army, red brigades, Palestinian Liberation movements, Action Directe, even Switzerland had a lot of things going on because it was geographically in the center of it. Could've he passed a deal to sell weapons in Switzerland and then suddenly moved ?
About the currents, I've checked the weather and winds that night with data that is still available. It says the ocean was very calm and there was almost no wind, the days before til the 2nd of jan. So I would more aim at the theory that either he didnt know how to swim, either at hypothermia, as you said. Also being drunk/intoxicated you can easily die in the water, even if you're a good swimmer. I heard dreadful stories regarding drowning, even in small pools, when you're under the influence.
About ATM's : as you said, people used it less back in the 80's. In Switzerland, we still travel with a lot of cash with us, and change it by hundreds or so. It's still a common thing to do, Swiss people like using cash and find it more convenient. A lot of my friends don't own a credit card and don't buy online.
I don't know if he was a spy, but it could have been the opposite situation. Swiss passport is very hard to get, but it is a great passport.
Not only Swiss per se. I Googled "international organizations in Switzerland" and of course, found an amazing number of them. In 1986, before the EU, Geneve was probably way more important than Brussels is today, because headquarters of virtually everything were there.
A very peculiar situation would be a citizen of any European country, non-European even, working in any position, from a secretary to higher up, in any of the international organizations headquartered in Switzerland in 1986. I think their passports were unique, allowing to travel freely worldwide. United Nations (Geneva office), World Trade Organization, World Health organization, international labor organization, part of Red Cross, CERN.. the list is huge. I don't have a link, it is just a Google list.
So he could have been killed for such a passport. With a changed photo and relocated to any other regional center for the same organization, he could have never been listed as "disappeared."
Another situation historically not atypical for 1986 would be a young gay man who left his home and country and moved, possibly to Switzerland, but it could be elsewhere. In 1986, AIDS was very well-known, and the stigma against certain groups, even in Europe, was high. So a man who leaves his (possibly, religious) parents in France, or the Iberian peninsula or Italy, and moves to Switzerland, can easily travel to Malibu for some NY festivities, alone. 1 Swiss franc in 1986 equaled around 1.6 US dollars, so they could travel. He gets killed, for whatever reason, or drowns. And if he severed the ties with the family, no one could be looking for him.