Problems with collapsed cells in tables

[JeremyRuston]

There are a number of problems with collapsing cells in tables documented here:

Here are a few examples that puzzle me, you can try those examples in TW: 

the 2x2 section is displayed as I expect here 
|!one|!two|!three| 
|>| 2x2 |1| 
|>|~|1| 

But when I try to move the 2x2 section on the "right" of the array, 
problems start to appear: 
what should I do? 
|!one|!two|!three| 
|1|>| 2x2 | 
|1|>|~| 

Then when I add a row to the previous example, a "fourth column" 
mysteriously appears: 
|!one|!two|!three| 
|1|>| 2x2 | 
|1|>|~| 
|~|x|y| 

Another example. This table displays only two rows instead of three 
|!one|!two|!three|!four| 
| 1x2 |>| 2x1 |a| 
|~|b|c|d| 
|e|~|~|~| 

And yet, when I add one more value in the last row, the table now 
displays correctly 
|!one|!two|!three|!four| 
| 1x2?? |>| 2x1 |a| 
|~|b|c|d| 
|e|~|f|~|