Hi there sorry to hear about your problem.
This can worry your pants off trying to get something going back in there.

Maybe you did not get the first insert installed in tight enough. They are depth specific to the tap made for that style insert.

I have great respect for Helicoil Brand products, even though, he did not specifically name them.
They probably make the kit for Napa to sell but don't hold me to it in today's marketplace.

Helicoil makes their coils from stainless steel and coil insert shape design is a time and experienced tested product.
In many cases manufacturers use them as "new threads" in the base material instead of tapping normal threads into it. They can be a stronger and a better faster thread in materials like aluminum or granular castings.
The stainless steel also helps breaks up any electrolytic bonding to both metals or coatings engaged.

Will740turbo appears to have dealt with this issue more than most people.
I'm glad to say I have not!
I don't spare anti-seize anything I assemble.

I agree that experience is the greatest teacher. I feel that every time a person has to put one of these into an engine, there are variations to the difficulty. Location being the biggest one.
At least, on an out in the open in-line engine, this works in your favor.

The idea of using grease to coat the tools is standard operating procedure. The air going into the cylinder needs just a touch more explanation, in case many might wonder, just how is that going to happen, with that hole full of drill, that greasy tap and chips.

What I do is apply air through an open intake valve using the intake manifold nipple port.
You can use compressed air hose if you have a compressor or the exhaust from the rear of a vacuum cleaner. Anything to get a breeze going through the throttle body will travel there.

I guess you could use the exhaust valve side if you have that manifold off.
I would not try to blow air up through any exhaust system. No telling what crusty creatures lurk down there from that direction.

May our thoughts and any good luck you need be with you!

Phil