Thunderbird Sport Spark Plug Threads Fixed With Helicoil (came Off)

Hi everyone. I need help with my TBS. I was changing spark plugs and found that cyl.1 sparkplug has been fixed with helicoil. The helicoil came out when i was taking sparkplug away. Tho it came nicely when screwing plug out. I buyed the sparkplug repairing tool with correct size (M12x1.25). Threads was already made M14x1.25 for the helicoil that came out. Thread tool will also make M14x1.25 coils for the sert that is used for fix this problem. So i bought M14x1.25 bolt just to check if threads are still there. I screwed this bolt in hole and it went there nicely 12mm but then it stopped. Threads in plug are 15mm long. What is best thing to do here? Do i use 12mm long sert and use it with good threadlock or should i use threadtool to make 15mm long hole for longer sert? I think it is not good idea cause it will cut more aluminium and could be that it doesnt go with threads (12mm) that are already ok with bolt. Only thing i wonder is that how this works if sert is 2-3mm shorter than original threads (15mm). Will it be hard to that plug of next time when needed. Some pictures added to make this easier to visualize

Tool:

https://www.amazon.com/AB-Tools-US-Pro-Rethreading-Inserts/dp/B075F8QB4G

 

On third picture, there is helicoil that came out