@ billy anorak59; thanks for the 1936 map. Interesting that he planted the line of trees on the far side of the ground in question. This suggests that the road was built as close as possible to the edge of the reason for the diversion and that there was not room for trees at the edge of the road. It really suggests that there was a pond there, or at least, boggy ground.
@derekdwc; yes, that seems reasonable. Your post does raise another interesting point. Like you, I always understood that he built the causeway as a direct route between Thornton Manor and the works, but most of the causeway is off that line; maybe he also wanted his own route to Birkenhead and Liverpool.
I've attached a map showing the most direct route: Red 'x's show that route. From the Manor, via his private road to the Brimstage road, near where the roundabout of the M 53 now is, then to Spital Crossroads, turn left along Church Rd., then The Wiend and Ellen's Lane to Greendale Rd. and to the factory. It's worth noting that he couldn't have used the shorter route: (blue 'x's) along Quarry Rd. and Quarry Rd. East, since that was the Storeton tramway (or maybe he had a private train)
I've been in touch with Unilever Archives to see if they have any info. They told me that the Archives aren't open to the public, so it's no use going. I'll post if they come up with any answers.