I'll try to answer; The conscious works roughly 16/7. The subconscious works 24/7 so let us start by what appears in nature to be the most important, is the subconscious. All life has a natural instinct for survival, but all life has a selective instinct too. It could be changing colour, hibernation or migration etc.
An intelligent species does not need either, but does need purpose, which in the beginning of life it would have needed variety to stimulate that intelligence. This of course would mean that life was evolving as one life. A one energy system producing its own pattern.) This is the only explanation for the design in nature. Yes I did say design; a natural one.
Getting back to the dreams; they seem to be coming from individuals, but if you insert the one energy, then they are coming from one individual system. Lincoln was in touch with the perpetrator, planning the atrocity, which gives the illusion of seeing into the future-a bit like a magician's trick. Helen dreaming as it happened needs no further explanation. Now compare the subconscious experience to a conscious one; We could do something that needing attention, today or three days later depending on necessity. Dr Donald Hoffman proved that all life cannot see accurately, and this is my way, but I guess many see it differently.
Best Regards
Dodin

Your dataset's too small to find a pattern here.

It takes more thn two to make a pattern. If you studied statistics, apparent patterns can from randomly by chance with no actual or real significance. This is why science demands that studies be replicated,peer reviewed and verified as valid data and not a random outlier.

Dreams, the Royal road to the subconscious. The fact is, we can only at this time have a subjective view of dreams, and there is no truly supportive evidence that there is any meaning to dreams. And therefore, are only the random firings of activity of the brain stem as it organises memories/images into linier data.
Lincoln may have had many dreams of his death before, disappearing like 'tears in the rain'. The same for Helen Duncan. Likelihood is, she was awoken during a ship sinking dream and thus, remembered it more vividly.

documented coincidences?

Yes, that they both saw risks.