Graham, thank you very much for venturing out to check for me. As long as they bear the weight evenly, I can't see why the boat shouldn't be supported along its keel - makes intuitive sense.
I noticed CLH have a loyal cohort of enthusiastic followers in tow on the forum.
They were very helpful across the phone today, but something doesn't add up. Once the final adjustments were made, it seems the boat now sits well on the middle central roller. The centre board could sit on the stern central roller. However, there's no way the bow is anywhere near the front one. And the measurement for winch height ensures the winching eye on the boat hits the lower rubber on the winch post.
I'll chat again to CLH and perhaps send them a diagram of what I think they said, to ensure my ear-to-spanner communication was successful. If it turns out I heard them right, I'll check with the yard.
Unless the front roller arm is actually the wrong one and too short, there really is no way to get the boat to fit as intended. Lowering the whole hull brings it worryingly close to the vicious corners of the agricultural wheel arches, so I'm not keen to entertain that approach!
Your blue rollers look like an excellent upgrade. Maybe something I'll fit once everything is adjusted.