TLC5926IPWPR Clock width
Your current clock is probably fine. You may want to slow it down a bit from an abundance of caution if the slower rate will not affect your system operation.

All of your assumptions and methods are correct as far as I can see. The datasheet values will have a some margin in them; it's common for values to be rounded to the nearest nice-looking number. Most customers are probably not as observant as you are (not that it's a bad thing!).

To precisely meet all of the datasheet requirements, though, it would indeed be safest to reduce your clock speed. It is probably not necessary. Will you be operating at 125°C (which is probably the worst case corner)? If not, that will probably help you significantly. 

Besides rounding to nice-looking numbers, another reason why datasheet values are not always self-consistent is that when the manufacturer characterized the value, the standard deviation is different between the two measurements. They may be able to control/measure the frequency of the test signal to a greater accuracy than they can control/measure the pulse width. If they pad the min/max specs by ±6σ, the different standard deviations can really change the published value. This only applies if the values are actually measured in two different tests; they may just be both derived from the same measured failure threshold and padded by different amounts.

If it's really very important that you keep a high speed and also very important that you know how safe you are, you could try characterizing the clock characteristics yourself. Buy plenty of parts/boards (hopefully getting a few different date codes), and for each part, increase the clock frequency until you start to see failures, running over your expected operating voltage and temperature range. Or if your order will be high enough volume (maybe at least $1M), ask the manufacturer to do this for you.