Both M-W and AHD include the listing "shown or showed," which would tend to indicate that they are both considered relatively common, although have "shown" in the first position may indicate that that is a bit more common. I would tend to use "shown," but I wouldn't find it overly jarring to hear people use "showed."

I probably default to show, showed, has showed when litteral showing has happened and has shown otherwise.

You have not shown us how you come to that conclusion. Maybe you could give us some example sentences.

Warum wäre man dann mit 'showed' offensichtlich stets auf der besseren Seite (#3)?

Ganz einfach, weil ich mich in #3 leider vertippt habe. Dort muss es 'shown' heißen.

I am amazed to hear that 'showed' as a past participle is ever considered correct.

Für mich hört es sich auch merkwürdig an, Hecuba.

editiert

Wenn außer 'Wiggeln' sonst nicht passiert ist, war es wohl kein traumatisierendes Erlebnis für dich. Schön zu hören, dass du damals ungeschoren davon kamst!