Table with colspan and rowspan in a guest-editable wiki
Mmm...
There is 2 different approach in terms of content editing.
The wiki approach with the wiki syntax (simple fast editing)
The WYSIWYG with the ck-editor (formatting and friendly tools)
What you want is to use the second one.
I didn't work with since a while and there are plans to upgrade it but I tested and it is still there.
See: https://doc.tiki.org/WYSIWYG
However I'm not sure it is wise to turn all your page to use WYSIWYG editor... As said long time not used... Good luck.