YA, MC could see strings of fate
I read this book in the 2010s from the library, it wasn’t particularly old. I’d hesitantly put the publishing date no earlier than 1990.
The cover of this edition I believe had a pair of green eyes on the cover. The main character had notably bright green eyes. Over the course of the book/series he learns he is descended from a race of beings that can see and manipulate the strings of fate. One example of this is of the battle between King Arthur and Mordred, where the swords were only drawn and battle started because there was a snake in the grass that they meant to kill. The swords being drawn told the armies that peace negotiations had failed and so they charged, with the leaders being too far away to explain the misunderstanding.
The other main thing I can recall about the book is a middle aged man character. He’s depressed, and has been some form of transmigrated into this world. He’s become quite comfortable in this world due to his knowledge of Earth technology (which I believe he sells? Or invents in some way?) but he has some sort of other goal. Maybe to go back to Earth?
I think the climax of the book involved some lady who was also from Earth or also had advanced technology, and there was some sort of battle or action scene on ships.