the wordlines only diverge after the decision is made
Which doesn't affect the original question at all. It implies two potential outcomes that co-exist. Otherwise, there's no divergence at all, i.e., no, they don't "diverge".
If divergence actually happens, then there are indeed two potential future timelines from a point just before the decision. Travelling into the future could potentially take either path. If it were not possible to take either path but only one path, then divergence does not in fact happen.
The problem with divergence --
In the next room over, while the President sleeps, a personal secretary is on the verge of a decision. An instant before the President awakens and decides, the decision is made by the secretary in the next room. Therefore, assuming divergence due to decision, two Presidents now making decisions result in four future potential timelines.
But down the hall, a Presidential aide has just recently chosen a tie from a rack of twenty ties. At least twenty potential futures came from that, so twenty personal secretaries gave rise to forty Presidents who created eighty potential timelines.
Given the number of decisions being made every instant among all sentient beings in the universe, time travel forward would be impossibly complex within less than a millisecond -- if divergence happened.
I'm pretty sure divergence is an irrelevant concept. (Hmmm... as I tried to decide what the right word was and then settled on "irrelevant", how many divergences happened?)