Ok, so another update. After at least a year of frustration and pain, I've made some progress AT LAST.
As far as I can see, there are several bugs in iTunes Match. Let's see if I can note down my observations.
1) Some files simply stop everything. iTunes Match doesn't like them and that's the end of it. Weeding them out is difficult.
2) There is no consistency in which files / encodings iTunes Match will and will not match. I have albums with 10 tracks, all encoded at the same time and in the same way and one will not match and goes into the magical "Waiting" state. There is no use guessing or filtering to try to work out which will and won't match.
3) When the magical "waiting" state is entered, *the file has been added to iCloud*. Yes, this means any subsequent attempts to add it will be seen by iCloud as "Well, we've got it matched and are waiting for it to be uploaded! Probably! - move on". I suspect the iTunes app then fails at uploading it, which holds everything up.
4) Uploading is glacially slow. Really so seriously slow it's incredible. Those who have followed my predicament will know I have to match 10% of my library which is failing. Right now I'm doing A, then B, then C in artists to get it matched. So of those 10%, I'm doing 1/26th (obviously not evenly distributed) at a time. A took 24hrs. B looks to take the same time. so 1/260th of my library takes 24hrs. Ouch.
5) There are still some files which will not match and will not re-encode to AAC. Why? No idea. Buggy AAC encoder, presumably.
6) deleting music from iCloud takes a long time also, certainly for these ghost files that iTunes Match thinks are in iCloud already but are not.
So the progress I've made is that I can now match/upload a good chunk of the remaining 10% of my library. Here's how.
1. Re-encode the problem files as AAC using default settings. (select them, right click, choose "create AAC file")
2. Delete the originals. Check "Delete from iCloud". Move them to the trash and EMPTY THE TRASH.
3. Wait a bit. Actually, iTunes Match should start automatically.
4. If not, select the files and choose "Add to iCloud".
The first "add..." might mark them all as "Removed from iCloud". That's fine. Just do it again. The files will probably be Uploaded or some might even Match.
Those that need to will start uploading. Very slowly. Boring boring boring.
I'm almost at the point where I think iTunes Match is worth the money - now I can fill in some of the gaps in the CDs I've been dreading to listen to.