That brand new swingarm you got will do the same thing at the cross brace. Sean at Braap motowerx came up with a fix that gussets it and extends the brace all the way through. Making for 4 welds and multiple gussets instead of the half ass work ktm did.
Now for the bolt. Heat it real good and try an air hammer.
Do not use sledge hammers and what not as you can very easily bend the frame or break the cases doing so.
This is a common problem in Hondas. Is what I found to work best it draining the oil and gas out of the machine and tipping it on its side.
Then I spray the penetrant lube on the one saide and have gravity force it down, I take a heat gun and blast it while I’m spraying it. Then I let it sit for a day, spraying it more every chance I get. Then I do that other side. After that I heat it all up and get it out with an air hammer. put the swingarm nut back in the bolt so you don’t destroy the threads