Thanks for the tips @Jamez0r.
I had actually already done exactly as you had suggested, though I wish i'd asked you earlier as it took me a few hours of trial and error to get to the situation that you have laid out above!
So I alternatively just tried putting the Timeline URP back in my assets folder and then pointing my project to the right place via the package manager, and it actually works exactly the same as when it's in the root packages folder. (as in same errors)
So at least it seems that it isn't just a directory problem, however that still doesn't solve my namespace issue, as I still can't build!
I have double checked that i've got the right versions, both are the latest (non-beta) versions from here: http://esotericsoftware.com/spine-unity-download#Updating-an-existing-UPM-package
If anybody else has managed to install these successfully, it'd be super handy if I could see that somebody elses namespaces are working and then try to work our why mine are borked!
Thanks again ʕ•ᴥ•ʔ