So are you going to do the Time Sert now? That is the only fix that works correctly. There is a reason it was the first response when asked what to do.
Hopefully the piston is not damaged.

 

I think old blu nailed it. Even though I'm a bit like you, OP, trying to get by. I've paid many a price doing that. (Insert self-kicking icon here)

 

As I last posted, I removed the incorrectly installed Helicoil and checked for damage. There's a limited view of the piston through the spark plug hole, and I don't have a borescope. I did not see anything amiss, and given the small amount of damage to the plug and insert, I had to assume the piston was good. I spoke to a friend who's knowledgeable and he felt like the piston would hold up better than the plug and insert if it were a contest.

I put a new insert in, using red Loctite instead of RTV. This time, I used a non-foul plug adapter to seat it, rather than the spark plug alone, to make sure the insert was flush with the top of the threaded hole in the head. Here's a picture of the set-up I used, just to illustrate how the original installation was doomed from the start. The red lines represent the new ID of the hole after being tapped for the insert. You can see that the diameter is too large for the tapered seat on the spark plug, so it went another 1/2" or so into the head before stopping.



Once the loctite was cured, I removed the non-foul adapter and installed the spark plug. When I first re-gapped and reinstalled the plug that had been damaged, it ran rough. I was told the platinum wire had probably been knocked loose, so I had to buy a new spark plug but it runs fine now.

The obvious issue now is "how long will it run fine?" I am selling this truck and letting whoever buys it know what I did, and they'll be the ones to find out. As with all other repairs, their life expectancy isn't certain.