The example with the rain is wrong.
It's either the proper "wegen des Regens" (Genitiv), or the new idiom "wegen dem Regen" (Dativ).
"wegen den Regen" means something slightly different (more like: "because of _multiple_ rainfalls")
There's a whole book by Bastian Sick (famous German author) named "Der Dativ ist dem Genitiv sein Tod." -- the title about the Dativ being the death of the Genetiv is playing with that idiom.
https://languagetool.org/insights/de/beitrag/dativ-genitiv-s... -- it's in German and discusses the (perceived) change of that idiom.
As much as I like Twain, the English language is one of the hardest European languages, when it comes to pronunciation (contrary to Italian, which sticks to a few simple rules). So, you're welcome, choose your poison.
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