I think that 19d leaving can be read as both VALE, i.e. farewell and also as RIA leaving - not being used. The fact that Valeria is a Shakespearean character was very misleading and had me very confused with completing the se corner. Tyrrell = Tyrone power without one + re for about and ll for evens, I presume.
Hello Ixion - I haven't really got an explanation either. A possible 21st would be Valeria. If you take away ria, you're still left with vale. I put it in because it's the only thing that fits. Sorry.
And I'm a dumbhead! I still think "ninsend" is better, even if there's no such place. I think 26D must be "Valarie" - anag of vale & air, although I'm not sure if there's a Valarie in Shakespeare. Just off now to check out Nunhead...
Here's a synthesised justification for 26d.
Valarie works both as an anagram ("possible")
of Valeria (the 21st character)
- or, parenthetically -
an anagram of ria + vale.