Being frank fella, you seem to be fudging a lot of stuff, do you feel you have the capability to be doing this work yourself?
You said the original attempt to drill it wasn't straight and M14 tap got stuck - redrilling all over again will have taken even more material away.
Did you torque the 18mm bolt to the required 70nm/52lbft +90 degree turn? even if you did, I would suspect it will already be starting to pull out.
Your new situation with the dog bone bolt having chipped a chunk out of the gearbox casing:
To fix it properly, retaining the current case, it would need the lost aluminium to be built back up with tig welding, re-drill and time-sert used for the best strength.
This would require the gearbox to removed because they won't be able to weld it that way up in the car
Plus with it being at the bottom of the car the porous alloy will have lots of contamination and be nightmare to weld.
You could replace the case, but suspect it would likely be cost neutral.