Thanks for the input!
I will go and amend the OP, especially removing all those things resulting in bad HTML.
EDIT: I updated the OP to incorporate your input. Sorry if I turned up many stuff resulting in bad HTML. I haven't really considered the impact this has on the resulting HTML code, but just tried out what can be done within the limits of the table code and how it gets displayed. But if it leads to unexpected results, it is something that should be avoided, even if it works on some browsers. So thanks for highlighting those problems.
Also nice to hear that there will be future improvements to the mobile site!