What does your dealer say? At this point, if the helicoils didn't work, and the dealer won't work, look at the possibility of going oversize on the bolts abit, seek advice on the best Loctite to use under the heat and vibration of the saw (they make 101 different flavours, and maybe even talk to a fastener specialist (ie fastener retailer selling only fasteners) to get a lock washer that will help. Apply all three at the same time, have a beer while everything sets up, and try it the next day.
Hope this helps.