I've installed without the K9 (anti twist) washers on one board, but I'm not entirely comfortable with it, just like you. No problems yet though. On other boards I either used the old screw (shorter) or just added about 2mm stainless washer above the strap instead. The K9 washer is in fact not very thick and it makes the pad and strap compress "locally" just under the washer a bit more, so I doubt it adds more than 2mm.
But on most of my boards I just mounted with both the K9 washers not worrying about that they intersect a little. That works too.
One thing: If you take out the screw from the strap (to fx add a washer), be sure to screw it into the strap as far as possible before trying to screw it into the board. Even with the new longer screw, it is good to have as much screw length as possible go into the plug before the strap contacts the board and start "resisting".