"there are 6 cells containing data in MouseX, and each contains 7 comma separated values. So I'd want a 6 x 7 numeric array.": Ths query has the solution, becuse you can have a matrix of 6X7. But your next statement, "It's also worth noting though, that there are a variable number of values in each of those cells.", if number of values are different in different cells, creating a matrix is not possible unless you are adding summy values in the rows.
csv = readtable("myStopData_MousePos_test_2.csv")
array=csv.MouseX;
array_cell=array(cellfun(@(x) ~isempty(x),array))
array_cell=cellfun(@(x) str2double(strsplit(x,',')),array_cell,'uni',0)
If all cell have the same array size, you can have that in a matrix
cat(1,array_cell{:})
