Having given it some more thought, what I should have said is that if I were in this situation (and I don't dare experiment with my chain adjuster bolts now!), I think I'd want to "exercise" the threads a fair bit before attempting the "screw in and saw off approach". That would mean, with plenty of oiling, being able to screw the bolts IN as far as they'll go (you said stiffly, but as long as they will go in all the way until the bolt head hits the end of the arm, that bodes well for screwing in the "remnant" later). Then, I agree, if you decide to go for it, screw out all the way until it starts to get too stiff (suggesting that a nasty corroded section of bolt inside the swinging arm has just reached the threads in the arm), then saw off leaving enough of the threaded "shank" section to cut a decent slot into for screwdriver work. It might be worth searching Youtube for items on how to extra broken off bolts, etc. There are plenty of videos on topics like that, but I doubt there's one specifically on this particular Ducati special challenge!
