I was of the same opinion on using a helicoil insert on such a highly stressed component like a crank. Problem is that there has already been one bad install on the flex plate end of the crank so it'll be difficult to gauge what effect it will have trying a repair like this.
The output end of a crank shaft is by far the most stressed point in a drive line and, as you can see, if you get assembling it wrong it will do some serious damage (hopefully it hasn't screwed up a seal or bearing in your transmission).

...and there's a reason why scatter guards are fitted to high revving street cars that get some strip time - so you don't get both of your feet cut off if your flywheel fails