Anyone see a problem with this method?
Just took up a couple of rows of laminate 2ft away from rising floor
area,put down a good bit of cardboard and then layed the underlay down then
the laminate planks again.
Its just the right height when it came to the rise area and they went down
flat over it so I'm happy with this but don't know whether it'll cause any
problems?

I've sort of come to a similar conclusion. I found the hardwood
doorbar not to be straight, and I managed to remove it, and use my
router on the back to machine a "lip" to go over the top of the
flooring. I then put the doorbar back with packing under one side to
level it.
Under the flooring I do need to raise the level one side though - I
have put down some 3mm hardboard sheet so far, but only on the run up
to the door. This makes the floor near level as it slides under the
doorbar lip.

This looks good, but I am worried that this will cause a reliability
issue with the floor joints, due to the mismatch in height between
planks.
I wonder if twinwall carboard would be better under the flooring, as
it would "give" more.

I have 3mm foam damp-proof underlay over the hardboard, so you can't
feel the hardboard when you walk on it.

Painters10