I would agree not to go with that welder.
There are plenty of competent welders out there though, so do not be so quick to write off the entire idea.
From the look of pictures of the cracks, that does not even look like the process I discussed with a local machinist who reports he has done many wheel widenings on wheels and never had any kind of cracking. According to the guy I talked through it with, the welds should be 1.5" on each side of the center of the wheel. Looks like kosman cuts and mounts the very edges, rather than using only the center core from the 9 wheel and using the bulk of the wheel from the donor blank. I am not a machinist, but that sounds like a big difference in load carrying capability.
There are plenty of welds on your bike already that, I presume, are holding just fine.
If you are going to bail on every idea that gets a bad report or two, then maybe it is time to trade out the swingarm and transmission.
Do what makes you feel right.
Let's just not jump on every alarmist naysayer bandwagon.