In response to the request for advice from an engineering expert I advise as follows.

Fork tynes are manufactured from steels that can be described as spring steel. THe most heavily stressed area is the heel because of the bend and change in the flow of stresses. The top radius at the bend is critical as too small a radius will result in a high stress concentration and the tyne will fail. And because this radius is critical damage to the radius can lead to cracking and failure.
Because this part of the tyne is critical it is essential that the composition and structure of the steel around this bend is uniform, and the heat treatment is also uniform.
It is virtually impossible to weld such a thick section together AND manitain a uniform composition of the steel and uniform steel microstructure. And unless there is this uniformity it is impossible to ensure a uniform outcome from the heat treatment.
Given forklifts are such high risk items the advice hence is that you would never repair a forktyne.
In respect of worn tynes, building them up is possible but once again the risks must be considered. Firstly why are the forks wearing? The forks should not be operating so they are incontact with abrasive surfaces - typically concrete floors. Drivers should ensure they never travel with the forks on the floor.
Finally straightening bent tynes - once again the risks must be considered. If they have bent at the heel then the steel there has already been heaviliy stressed so the risk of subsequent cracking and failure is much higher. Straightening can only be done with appropriate heating and then heat treatment and that can also increase risks of failure.
And once again why have they bent - in normal use they should never bend. Mostly bending results from either using a fork on its own (without reducing the load by 50%) or the fork tip is being used to move loads without recognising that the increased load centre may double the stress on the fork tyne heel. Both situations require serious risk assessment.

Hence this engineers advice:
1. Make sure forklift tynes are not abused and they should last the life of the unit.
2. And if they crack, bend or wear more than 10% due to abuse, replace them with new units.