Genesis 1:1-5 — Day One

The reduction in the earth's rotational speed is not the reason for adding the leap second (or at least not the primary reason). The real issue is that the definition of the second is not exactly the same as one mean solar day divided by 86400.The mean solar day is actually about 86400.002 seconds long based on the current definition of a second. So independent of the fact that the mean solar day is lengthening at a comparatively small rate, the 0.002 sec between 24 hours based on the definition of a second, and the mean solar day piles up to be about 0.9 seconds in something like a year and a half. At that point a leap second is added.The rate at which the difference accumulates is increasing by that 15-25 millions of a second each year, but as you've pointed out, that is a significantly smaller effect.The confusion between the two effects was the basis for a silly PRATT that predicted that the earth could not be billions of years old because it would have stopped rotating by now based on the rate at which leap seconds were being added. But as discussed above, the leap second is not caused by the lengthening of the solar day.We can find the history of the definition of a second, and some of the factors affecting the length of the solar day in the wikipedia article linked to below.Leap second - Wikipedia

Are you discounting the last verse of chapter 4 as being inaccurate? I don't understand the reasons you give for not including that last verse as part of Genesis. I don't understand any of your reasons, but this one seems totally irrelevant.