No 12648, Monday 10 Jun 2019, Skulldugger

Tough one from Skulldugger.

ACROSS
1   Idyllic setting outside earth (6) HEAVEN {H{E}AVEN} &lit
4   Spell fairy put on old duke (6) PERIOD {PERI}{O}{D}
9   Air to paralyse fish (4) ?A?E {Addendum - DACE (~daze) - See comments)

10 Seasoning as amenable as salt (4,6) ABLE SEAMAN*
11 Rod is involved in anarchist plot (6) PISTOL {P{IS}TOL*}
12 Oldie's prodigal redhead son grunts (8) SOLDIERS {OLDIES}*{Re...d}{S}
13 Much that might be had at flea market (5,4) GREAT DEAL [DD]
15 Thin layer with fairly regular mass (4) FILM {FaIrLy}{M}
16 Intend to traverse the Northern Alps from the east (4) PLAN [T<=]
17 USA mandate to repeal immigrant's right repulsive to a sickening extent (2,7) AD NAUSEAM {USA+MANDAtE}*
21 Boeing leak that can be traced to criminal? (8) CONTRAIL {CON}{TRAIL} I don't think it's a leak

22 Instruments with unexpected drawback returned to Operations Research (6) ORGANS {OR}{SNAG<=}
24 Nancy's father finds politician (conservative) imperious (10) PEREMPTORY {PERE}{MP}{TORY}
25 Sound level for swimmers (4) RAYS (~raise)
26 RCMP must excel in this Canadian political constituency (6) RIDING [DD]
27 Stopped, picked up, impounded (6) CEASED (~seized)

DOWN
1   High achiever leaving college with additional burden (7) HEAVIER AcHIEVER*
2   Competent chief of army division briefly (5) ADEPT {Army}{DEPT}
3   Sanctioned drug nurse administered initially reduced body fluids (7) ENABLED {E}{Nu..e}{Ad...d}{BLED}
5   Exam cleared even by unintelligent student annulled with little effort (6) EASILY {ExAm}{SIlLY}
6   Foolish remarks asinine, beastly, it must be admitted (9) INANITIES {ASININE}* over {IT}
7   Local street rally with back-up plan (7) DIAGRAM {MARG}{AID}<=
8   Family acted on old Don's behalf (5,3,5) FLESH AND BLOOD*
14 Discrimination for a time interrupted state of equality two people had briefly (9) APARTHEID {A}{PAR}{T}HEID Anno pending (Addendum - {A}{PAR}{T}{HE}{I}{'D} - See comments)
16 Leading revolutionary embraced by colleague a trailblazer (7) PIONEER {P{NO 1<=}EER}
18 She follows; Lawrence, a short, callous Yankee leads (7) ACOLYTE {TE} after {A}{COLd}{Y}
19 Cross recklessly one day around mid-morning (7) ANNOYED {ONE+DAY}* over {m.rNi.g}
20 Female boxing program to materialise (6) HAPPEN {H{APP}EN}
23 Shoots shell from gunwale on ship (5) GERMS {Gu...lE}{RMS}

Reference List
Old = O, Duke = D, Son = S, Mass = M, Politician = MP, College = C, Drug = E, Lawrence = TE

GRID