Page Layout
Orientation: Landscape
Paper Size: A2 420x594
Scaleto: 144%
Roll width: 24inch
The reduce/enlarge box is ticked with 'fit to roll paper width' selected
The optimize enlargement is also ticked
Is this what you meant?
Well, I didn't think that it was but, after trying to configure my 4880 with analogous settings, I think that you should set
Page Size to A3
Output Paper to Same as page size
Untick the reduce/enlarge box
Untick the optimize enlargement box, or at least recheck that you actually want this on
Then, in the LR print module layer tab, use the height and width sliders to set the cell size you need. You can also set the margins to position the image on the page.
By doing this, you should be able to stay in LR to configure your image rather than having to use the driver settings. I have a number of presets for each paper type with various page sizes so that I can be confident that the driver has the settings that I need without looking at them.
I hope this helps,
Best regards
Alan
Edit: I don't really understand what is going on with your original set-up, but it looks at lot like the problems with double profiling, with both the printer and LR trying to control the image size. Alternatively, perhaps its a lot like lying to your doctor.