I dont know much, but it may not be available at that quality (it might be available at lower quality, for example mp3 instead of flac). If the issue persists, you probably have to look somewhere else for them, yes. I think I had around 4% error rate on my playlists. (Many of which I havent yet downloaded, too bored to look them up somewhere else, lol.)
I had a notepad opened ~at all times to keep track of what is downloaded and what not.
Happy that you eventually made it, this is where the fun begins😄
Yes, I think that's what I would do. Downloading from deemix and tagging in other programs are separate processes, so you can do them separately at different times.
I'd probably even encourage to download them now, because as you can see, deemix doesnt get updated anymore and things start to break.
It also gives you time to revise your methods of downloading and tagging. (For example, I started downloading my songs from a website, one by one, until I found deemix (that website got shut down soon after, rip).)
So, no need to do them simultaneously. Download them now and tag them next year or whenever you can. You can afterwards simply sync the ("properly") tagged files to your other devices and have the metadata everywhere.