Well, If it were just the mayonnaise, it would be easy. I’m sure you already know all this, but for future visitors to this question: mayonnaise, like hollandaise and bearnaise, is an oil-in-water emulsion, with microscopic droplets of oil suspended in the water phase, with lecithin from the egg yolks stabilizing the mixture. The higher the oil to water ratio, the thicker the mayonnaise. A split mayonnaise occurs when the ratio gets too high and the droplets of oil break out of the mixture and merge together. To rescue a split mayo, you simply start a new mayonnaise with an egg yolk and a bit of water, and then slowly blend the split mayonnaise into it, re-establishing the emulsion.
Now, usually when people are rescuing mayonnaise, it has just recently split and is not particularly separated; as a result the oil and water end up getting mixed into the new mayonnaise at about the same rate. If one were to accidentally add all the oil first, the ratio of oil to water could become too high, resulting in the mayonnaise breaking again. In contrast, adding all the water first would be fine.
So if you were to separate out as much of the oil as possible, beat an egg yolk and a bit of water, slowly beat in the watery post, then slowly beat in the oily part, I think you’d end up with a stable mixture. The open questions would be, would the whitefish and other solid ingredients interfere with the re-emulsification? And does the texture of the spread rely on an oil-to-water ratio which is not achievable without industrial emulsification equipment? So I’m not sure whether it is possible to rescue your spread, but that’s how I’d try to do it.