A cute puzzle by Otterden; the rubric reads:

Each of the four sides of the completed grid contains three proper nouns of a kind, not further defined

which added to the frisson when I started attacking it.

  • 9ac: KID GLOVES: Soft digital covers Boy George adores: KID + G(eorge) + LOVES
  • 10ac: TOTEM: Emblem of a form of betting – the first of many: TOTE (ref. The Tote) + M(any)
  • 11ac: SCANNER: Comes across critic after opening of show: CANNER (= critic) after S(how)
  • 12ac: RECLAIM: Recall miracle play: *(MIRACLE)
  • 13ac: SOU: Resources show very little money: reSOUrces
  • 14ac: FORESEE: Anticipate front view: FORE (= front) + SEE (= view)
  • 17ac: MISSION: Avoid one on delegation: MISS + I + ON
  • 19ac: ARTICLE: Credential unfortunately leaving out end item: *(CREDENTIAL - END)
  • 22ac: HENPECK: Continually criticise bird weight: HEN (bird) + PECK (? imperial unit of volume)
  • 24ac: NEY: Say no to old marshal: ref. Michel Ney
  • 25ac: ANISEED: A need is to provide liquid flavouring: *(ANEEDIS)
  • 26ac: PICKAXE: Choose to dispense with tool: PICK (= choose) + AXE (= dispense with)
  • 28ac: RAVEN: Hungrily devour bird: Double Definition
  • 29ac: SWEETENER: Come on in – coffee perhaps?: another Double Definition, and a good one; “Come on” and “in coffee perhaps”
  • 2dn: INDIA: Country favoured aid redistribution: IN (= favoured) + *(AID)
  • 3dn: UPLANDS: Good periods getting to secure higher areas: UPS (= good periods) around LAND (= secure)
  • 4dn: OBVERSE: Heads face up to old boy's poetry: O(ld) B(oy) + VERSE
  • 5dn: LUSTRUM: Crave spirit after five years: LUST + RUM
  • 6dn: SHTICKS: Supports include hospital attention-seeking devices: STICKS around H(ospital)
  • 7dn: OSTRACISE: Review riot cases ban: *(RIOTCASES)
  • 15dn: RETRIEVER: Trier somehow always one to get things back: *(TRIER) + EVER (= always)
  • 16dn: EEL: Devious one up to be a general: LEE reversed
  • 18dn: ICE: Reserve kitchen regularly: kItChEn
  • 20dn: CLEAN UP: Reduce mess list in vessel: LEAN (= list) in CUP (vessel)
  • 21dn: END USER: Final recipient endures being out of order: *(ENDURES)
  • 22dn: HYPHENS: They join up with egg producers who are after detailed promotional material: HENS (egg producers) after HYP(e); it took me a long time to work out that detailed was the indicator for de-tailing “hype”
  • 23dn: NECKTIE: Cheek to marry in formal wear: NECK (cheek) + TIE (marry)
  • 27dn: ANNUL: Cancel annual outing away: ANNUAL - A(way)

I guessed the perimeter theme fairly early on, but for some reason was working on LONDON/LISBON rather than BERLIN/TEHRAN for the bottom right-hand corner for rather too long.