You will find the Douglas Radiadraulic leading link forks a lot stiffer than they look. The frame on the on the other hand is more limber than it looks! Though for such a strong fork, I always wondered why they used such a cheap bit of sheet stock for the upper yolk. The fork was a favorite of the grass track boys, who broke up many a Mark model just to get the front forks. They had a lot of travel for the day (pre-telescopic forks), and held up well to the rough stuff. The main drawback was the lack of effective damping.
Unless your front spindle is loose, both sides go up an down in unison without much worry of the wheel tipping side to side. If it is bent at the top, the wheel may permanently be tipped over to one side, yet pass the string test. Set the bike up vertical, and stand well off to the front and rear. Often you can sight if the wheels look 'out of plumb', or if something looks a bit odd.
I've not seen figures for the rake or trail of the Plus models. The frame is different in other aspects to make them definitely Plus or Mark. And the forks are different too, in that the brake anchorage point is a inch further up the tubes than a Mark, the removable headlamp brackets, and stiffer springs (not taper ground.) But I will hazard a guess that the headstock geometry is the same, till proven otherwise!
-Doug