I suspect it wouldn't matter how much time that driver had because he wasn't looking properly and kept turning long after DHD was visible.Only 'nowhere else to go' due to the excessive speed. If DHD had considered the situation (wet road, single lane highway, junction and other road users) and heeded the 'SLOW' warning that was painted on the road in big, white, well defined letters, he would have had a lot more time to think through the situation. The driver would also have had more time to spot him.
And that's still making various assumptions about the speed. The meaningful question is whether it was a reasonable speed. I felt it was, but there's always someone on this site saying one could cycle slower or use another route and get up earlier or get home later in order to accommodate the extra time needed.
And yet it's a "mistake" that motorists make often.[...] but it would seem the vehicle was showing many of the signs that suggest it was about to turn, and me personally having just entered the scene at that kind of speed from around a hidden bend would be wondering if the driver had seen me. Assuming they had would be a mistake.
Was it "a hidden bend"? I don't remember a reverse-angle shot other than the one from inside the turning.
"About to turn" is not the same as "turning" and I would expect most experienced cyclists to have discovered that slowing for motorists "about to turn" sometimes encourages them to turn across you regardless - it could equally be that DHD's slowing/braking encouraged the Merc driver to turn, although I still think it's more likely that they didn't see him because they were looking only for motor vehicles.
Anyway, we can't really discuss this until the cows come home because tough questions just get cut out and left unanswered.