What are the requirements for getting the Summerset Shadows quest for the Windhelm Fence?
Solution 1:
Windhelm, Markarth, Solitude and Whiterun all have "influence" quests that each become available once you've done 5 of the randomly-generated jobs from Vex and Delvin in that city. Since jobs in Riften or cities you've already completed do not help your progress towards completing the influence quest, you can save before taking a job from Vex or Delvin and then reload until you get a city you haven't completed yet, if you feel so inclined.
Completing the influence quests unlocks fences in Markarth and Windhelm, but most importantly, each influence quest you complete increases the available gold pool of all fences in the world, up to 4000 once all quests are complete.
When you've completed all four influence quests, plus the main Thieves Guild storyline, you can become the guild master, receiving a significantly-upgraded suit of the guild armor you gained when joining.
Solution 2:
Trigger Thieves Guild quest - Summerset Shadows Summerset Shadows given by Delvin Mallory of the Thieves Guild, who instructs to speak with Torsten Cruel-Sea of Windhelm. Quest is not added and speaking to Torsten presents no option to talk about the quest.
Reason: Linwe, leader of Summerset shadows was killed in an earlier "Kill the Bandit Leader" radiant quest, causing Summerset Shadows to bug and not start.
To fix:
Do not speak with Delvin first to start the quest. Preferably, leave one or two Thieves Guild Small Jobs quests unfinished by not speaking to Delvin or Vex to complete them yet. I left one done but incomplete by not speaking to Vex before following the steps below.
Go to Uttering Hills Cave - coc UtteringHillsCave01
Kill existing bandit chief there.
Replace with Linwe using code - player.placeatme 0007d679
Go back to The Ragged Flagon to complete all the necessary Small Jobs quests. I went back and spoke to Vex to complete my one and remaining Small Jobs quest.
Then speak with Delvin, and Summerset Shadows quest will be added.
This is the only solution for me.