Your point: "If the Maya astronomers made very careful observations over long periods of time, and if they were able to establish that 12 Venus years are slightly longer than 7007 days, and 15 Mars years slightly longer than 11699 days, they could have drawn up a further pair of excellent sequences" can be stated in stronger terms.
There is no doubt that Mayans wrote down Venus tables.
Concerning Mars tables of 78, 360, 520 and 780 days there are questions. Questions arise from mythic story lines that cloud aspects of the Venus and Mars table.
The most critical question to solve is, what form or forms of exact mathematics were used that created planetary, lunar tables and eclipses of many types?
Vedic and Chinese mathematical astronomers used the Chinese remainder theorem (CRT), a form of math that C.F. Gauss proved by his famous 1801 book "Discussions on Arithmetic". The power of CRT to create planetary and lunar tables, and predict solar and lunar eclipses are well documented in India and China.
Floyd Lounsbury worked on solving the Mayan mathematical astronomy by a culturally unique CRT. Two of Lounsbury's papers are linked in this thread.
One day, someone will complete Lounsbury's quest, and return Mayan mathematical astronomy to its rightful place in history, as an equal to Vedic and Chinese mathematical astronomy.