I'm thinking you have a bad Thunderbolt cable here. It's the only logical thing that fits the fact your displays fan & MagSafe cord are powered. It turns out you can plug in a Thunderbolt cable into the external ports and the display into your Mac. See what happens then.
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=== Update (11/26/2017) ===
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mmm... You do have me scratching my head here. You have removed the usual suspects.
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At this point it needs to be something on the logic boards power circuity that needs attention. This is beyond a DIY as you need to have access to the schematics and understand how to troubleshoot at the component level. Even then you'll need to find a replacement part and have micro-soldering skills.
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The other direction is to just replace the logic board while expensive it's a lot easier!
I'm thinking you have a bad Thunderbolt cable here. It's the only logical thing that fits the fact your displays fan & MagSafe cord are powered. It turns out you can plug in a Thunderbolt cable into the external ports and the display into your Mac. See what happens then.