Willy, you don't say which handbrake extension you had I'm guessing not the Kobran, but you raise an important point.
On a x250/290 without a swivel the handbrake is not too bad, but the swivel changes things, at least for me. After driving commercial x250's the first one I drove that was converted was a WildAx and I was appalled at the handbrake operation, I traced this down to two reasons. First the swivel raised the seat, and second they bent the handbrake attachment to clear swivel!
Our Globecar has a much better arrangement, the swivel doesn't raise the seat as far, and it doesn't need the WildAx bodge, but I still prefer the Kobran attachment.
As for fouling the seat when swivelling, depending on seat position it can nudge the seat back rack knob, this won't release the handbrake, but can sometimes adjust the seat back.