Yes~ what annoys me is, that some requisitions are far back in the queue and when you finally get to them, you can't finish them anymore, because the enemies that would give you said items are long gone
For example the Decorative Gems, which you need for the Hinterlands Tapestry Requisition, are only lootable from the Templars in the Hinterlands, but I got that requisition after I already had killed all Templars there. 
I found a nice work around by using quick save/load for that one.
If you go to hinterlands, at the Dwarfson's Pass camp, and walk west till you hit that lodge near the top of the hill, there should spawn 3 templars every game play.
Game play as in, every time you log in to the game, in other words, if you exit to menu and log back in, thats counted as 1 log in. The same goes for quick save/load.
Just quick save when you are there, kill the templars and loot, if you get some of the items you need, quick save again, and then quick load. Now, just rinse and repeat till you have all the items.
This works for all the requisition's that you originally couldn't finish, but find out there actually are a few that still respawns each time you have logged out and in.
Btw, thank you so friggin' much for telling about the JoH throne counting towards 21/21, I had a small inkling that it might have been the case, but I have not gotten that far into the game yet as to actually test it.
And as you, I too, have the same problem of being at 20/21 with all the thrones and decor's, except for the JoH throne.