timing constraint for clock mux and how to set output delay

The Fast I/O assignments don't work on DDR, so you have to use the MegaWizard/IP Console to create the altddio megafunction, and once done it will always be put in the I/O cell. This is unlike a SDR register, which can be done in RTL, is put into the I/O cell by the fitter, and the assignment can therefore tell the fitter what to do. I'm guessing with the assignment it either looked for an RTL register and couldn't find one to put into the I/O cell(not even check if DDR was used), or it found one but couldn't put it into the I/O cell because the DDR was already used. Regardless, it's dumb to say it was ignored.

I do think the Fast I/O assignments should be deprecated, as there's little use for them nowadays. (Once upon a time that was the only way to get them used...)

A quicker way to check is go to the Fitter Report, Input/Output pins, and there is a column that says whether the input or output register was used. (Although it is nice to physically see it in the Resource Property Editor, as your screen-shot shows). And if you have timing constrains, report_timing will show it going to/from an I/O cell too.