I don't think the crankcase pan is anything trivial to remove on the W164. I briefly considered removing the felt liner/insulation on mine out of fear of soaking it in motor oil upon removal of the drain plug. IIRC cross members and steering linkage would have to come out. Bluetec. Decided a $100 oil extractor was cheaper.
As for time, effort, and reliability of a Helicoil(tm) or similar repair? Don't worry. In critical applications we often put a Helicoil in BRAND NEW because Helicoil threads are stronger than the surrounding material. Especially when using a stainless steel bolt in an aluminum plate. Any competent mechanic/machinist will take more time lifting your car, more time removing the access panel underneath, than it will take to fit a Helicoil.
Thread-Sert(tm) is the brand I usually use.
Am sure I'm committing an act of heresy to suggest this, and think it will take longer than the Helicoil, but consider use of JB Weld epoxy. Cut the insulation back a bit. Strip paint off the pan and possibly the bolt with wire brush or sandpaper. Clean all the oil residue off with carburetor cleaner, mineral spirits, or similar. Glue the bolt in. Not the threads but seal around the bolt head. And from now on use an oil extractor from the topside.