You are correct; a rolling “x” stroke will not properly sharpen a blade. In fact it will only sharpen one side of the bevel. A straight up and down stroke will not work either.
A warped razor will only roll on one side (so you can touch all of the edge on that side). However when the blade is flipped, the middle of the edge will not touch the hone… only the point and the heel.
In fact, a great way to tell if a blade is warped... compare the spine wear and bevels on both sides, if they are uneven, chances are the blade is warped and may be difficult to hone for the inexperienced.
There are two ways to sharpen the blade
1) A narrow hone 30-40mm or A about 1 ½ inch wide is perfect for most warped blades, and works just fine for un-warped ones too… of course the edges of all hones should be rounded, but I recommend only slightly more rounded than normal for the warped blade (more rounding is not a problem for un-warped blades).
However some find it difficult to keep the blade flat on a narrow surface… but in my humble opinion this can be solved with practice… indeed most of those (expensive) vintage German Water hones (the ones in the hinged box) are very narrow, and folks in the old days used them quiet successfully to sharpen razors.
The other way is difficult to explain but rather easy to do in practice.
With the availability of wide hones “today”, we tend to use the middle of the hone most of the times. However when dealing with a warped blade it is best to work close to the edge of the hone… as the blade falls off the hone during the final part of the “X” stroke, the edge of the hone will have a chance to touch the middle of the blade that normally does not touch on that side of the blade (the concave side).
So you may roll one side of the blade (convex side), but the other side you bring down the tang and allow the edge of the hone to "follow" along the edge of the blade... (or to put it another way... Allow the edge of the blade to "ride" the edge of the hone).
(*Caution* you are not actually using the very edge of the hone... you are using the surface of the hone close to the edge of the hone to contact the middle part of the blade as you bring the tang down... it is a practiced movement, to contact the hone but not bring the tang so far down that you are actually honing on the edge of the hone... as a guide, watch the "black" on the surface of the hone to see how close to edge of the hone you are contacting).
If you perfect this method, not only will you be proud of the resulting edge, but you may never get those unsightly “uneven” bevels or spine were when you are done (that is: if they weren't there, or were very small to begin with, or you removed the hone were during the restore).
Bart made a Wiki entry in SRP explaining this some time ago but I cannot find it at the moment, but I will post a link when I find it (incidentally I do not see it in the Sharpening Academy).
Hope this helps my friend.
EDIT: found it: