Wasn't the length of a cubit adjusted every time the Egyptians got a new Pharoah?

Right. The cubit varied from place to place and time to time, but it was always defined as the distance from the tip of the middle finger to the elbow. Typically about 18".

Lakh is related etymologically to the lox of bagel with lox. The IE word originally referred to the fish but became applied to other things as the word spread beyond the range of the salmon. Sometimes it was another fish but it also was applied to the color of the salmon and became part of the word lacquer and some large number, e.g., 10,000 from the fish's habit of coming in large groups during the spawning season.