Cell merging is done only in grid UI; the data store is not affected.
Column sorting and cell merging by index are incompatible, because sorting is store's job, while merging is grid's job, and after sorting, grid can not get the mapping between the new row order and the old one.)
Column sorting and merging by id are compatible.
To get currently merged cells, use grid.getMergedCells();
The merged cell will use as default the content of the cell at the leading edge, if major cell is not indicated.
Note: To see the tundra theme, just append ?theme=tundra to the URL.