Hi, you have a typo in your DDX roatate90="90" must be rotate90="90"
I am not sure that the tagging is the problem, the DDX works with tagged and untagged documents here.
regarding 1.: The watermark must be associated to a set of surfaces in a PDF tag which is done in the way you have done it. I don't think you can do it more simple than that. regarding 2.: Not that I know of, but I also don't think that tagging is your problem. What kind of PDF are you using? Do you have dynamic XFA?
There may be other reasons why the Watermark cannot be placed. Protection, dynamic XFA PDF (which gets rendered with XFA plugin). Which version of AEM Forms are you using?