Q: How can I select photos from iCloud in other apps when this option has been disabled?

I am fairly certain that the reason is that you have space enough. I have about 600 GB of pictures in iCloud and my optimized library is only 65 GB on the disk, and this is the situation that the article describes: that you can only see the photos that are in full size on the disk, which for me is about 3000 out of 48000 photos at the moment. Interoperability between programs seem less important than making iCloud actually function right now, but we can hope for an upgrade at some point.

To get a photo into the photo browser, I can probably "just" do an edit on it, which may bring it to the computer (but I am not certain), and then it may or may not appear in the photo browser. I can also just make a folder for photos I need for whatever project I need them for and then export them from Photos, but this is really stupid double-work, when the data is already present. Very unlike Apple I should say.

I do not get any error messages, btw, I just can only see that the number is incorrect.

My system is in Danish, but that does not make a difference according to HT204669.

The tradeoff is not acceptable for two reasons:

1) the same functionality works as intended on IOS, but this functionality is not implemented on MacOSX. I was just working in Pages on my iPad and noticed that I have all 48173 photos available, even though there, I cannot search for anything (which is odd, but definitely better than not having access to anything at all). In Pages on my Mac I can see that I don't have the same photos, so I guess they are the same 3000 that I look at in Mail. (it does not show me the number). 

2) The data about the full size photos is fully available to the Photos App including metadata to search and thumbnails to show, or else it could not work, and this was always used in the iPhoto setup. The photo browser should just be a subset of the browsing done in Photos, but someone may have forgotten about it. Metadata is what you can search on, like place or name of the photo or comments etc. They are readily available in Photos - and the search in Photos is much improved over iPhoto, just to say something good too, and spending time doing your metadata straight away is really paying off when trying to find a picture from 5 years ago.

I tried to select a photo and double-click on it, but it did not turn up automatically. I suspect that it is just downloaded to the screensize and that is not enough for it to be placed on the computer - or alternatively, that I have to restart mail for it to work.

I do actually have an external disk with all my photos in Aperture, as I use it to import to and to get rid of the bad ones before geotagging them in HoudaGeo and importing them in Photos - I do bracketing for all photos - I am on 2Mbit upload, so having 5 GB to many photos to upload takes days , and I want to avoid that - so that they should be available everywhere, but this "feature" breaks the promised land.

In Music on my iPhone and in iTunes on my Mac, I can choose to see either everything in my cloud-library or only the things that are available offline. This choice should be available in Photos too. They probably even have the code for that to share.

The tip about using Photos to start the process may be the best hint yet, though, but it really shows how much is still lacking in terms of thought through functionality.