Everyman: A Few I Can't Parse

[chantaspell]

15a 'Gone out to lunch': register describes one; that's open to question (10)
It must be negotiable....but why? Clear explanation please coz I'm not great at these!

13a in their oddly endless hollow time (7)
Interval....so it's "in" + the odd letters of their...and now I get confused. Is endless hollow Vale without the e? Vale = hollow? Who knew?

27a 'Atonement' writer McEwan regularly lost, confused (7)
It's penance, but why?? Pen is writer.....then?

28a Air essentially unchanged for centuries!? Environmental group reportedly splits (12)
if it's greensleeves, what's the "essentially unchanged for centuries"??

many thanks!!

[Mamyalynne]

15 Gone, anag + i (one) in Table

[Lie-in King]

15a Negotiable - anag 'gone' + 'table' (register, describes; is round) 'i' (one)
13a Interval - 'in ThEiR (oddly) val(e; endless)'
27a Penance - 'pen' (writer) + anag '(M)ce(w)an' (regularly lost)
28a Greensleeves (air; tune) - "greens leaves" (reportedly)

[cashier]

13 Alt letter of their are ter and endless hollow val(e)

[klazomaniac]

'Gone out to lunch': register describes one; that's open to question

gone anagram = nego + table (register) describing (taking in 1 (one)

[Mamyalynne]

28 An 'Air' is a song and this one is very old and unchanged.

Sounds like - Greens leaves

[Toorak]

Anagram gone & I inside table (register)

Pen (writer) & anagram (m)ce(w)an

Greens (environmental group) leeves (sounds like leaves/splits)

[chantaspell]

Thanks all!! Not too happy with Vale meaning Hollow....but I'll suck it up. Quite a nice Everyman today, I thought....especially compared to a few weeks ago.