I don't know what photoshop uses them for, but, child images with SubIFD tags are quite valid and typically used for embedded thumbnails.

I absolutely consider the first one a bug or at best a useless error message. I initially thought it was complaining about the colour profile in the image I was trying to print. I had no idea the error message was complaining about a colour profile it wasn't going to use and having turned off profiles because I was printing profiling targets I sure as hell didn't want to risk messing up the colour processing.

As I said it took me an hour of messing with the profiling software and different images and printer setups before I discovered what the problem was. Yes then I could and did load each of the printer setups I created for profiling and selected a valid ICC profile it wasn't going to use then turned off the profile again and saved the setup. The ICC profiles became invalid because I replaced them with new ones I was generating.

First a minor one which I emailed about and didn't get a response so I assume it was not received:

After tidying up some system ICC profiles Qimage started complaining on printing about invalid/corrupt colour profiles and telling me to check profiles without giving me any clue what profile it was complaining about.
After messing around for more than an hour it turns out that with printer setups having printer profile (colour) turned off Qimage still checks and will complain about the profile that was selected before it was turned off.

Second is from a thread I saw on DPreview about image dpi in tiff files.  It never caused me a problem because I never yet relied on tiff file resolution, however, Qimage is getting it wrong.

two tif files saved from photoshop give this:

exiftool -a -G1 -XResolution "be.tif"
[IFD0]          X Resolution                    : 360
[SubIFD]        X Resolution                    : 180
[SubIFD1]       X Resolution                    : 90
[SubIFD2]       X Resolution                    : 45
[Photoshop]     X Resolution                    : 360

exiftool -a -G1 -XResolution "ad.tif"
[IFD0]          X Resolution                    : 360
[SubIFD]        X Resolution                    : 180
[Photoshop]     X Resolution                    : 360

Both images are 360dpi. Windows file property details, faststone, XnView, and Fastrawviewer all show 360 dpi while Qimage appears to be taking dpi information from the last SubIFD it read.