Update (yes it's taken this long to get an update from Tesla service)...
Tesla sent this car over to a local body body shop where they disassembled the rear gate area to take a look at what was going on with the channel near the spoiler. Turns out it's 3 separate things.
1) someone allowed a ton of weld spatter to remain on the car before it went to e-coat (body in white). The 30-years-doing-body-work-shop-owner doesn't understand how there's so much weld spatter there since Tesla welds using robots. So he's like "maybe they had a guy hand weld this?"
2) the rear window was seated incorrectly which clogs the drain holes. He says this happens to a lot of Model X. I can't find any evidence of this posted on TMC but whatever. He said there's a big bead of urethane that goes around the rear window. If the window is seated poorly or they use too much urethane, then the urethane gets down into the drain holes. In my case, the window was mounted 2mm too low. He said it's actually lucky (lucky?) that I found this problem since the window is crooked a bit too and it's just terrible. He questioned why I took delivery of this vehicle because... it just is very sub-par.
@bmah - the reason is because I am an idiot and the dumbest MF-er on TMC.
Anyway, the urethane effectively clogs those drain holes and results in that "lake effect" where water won't drain properly out of that channel below the spoiler. So yeah, if your car is new, but you have to use a leaf blower to empty this channel, you're probably dealing with the same issue.
The only way to fix it is to replace the rear windshield after removing all the gunk.
3) body shop owner said they see more weld spatter in the drain holes. He can't figure out how this is happening but he says it's just Tesla being Tesla with their famed low-end build quality lololol.
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Edit: Oh by the way, in case you're wondering why Tesla won't just replace the entire rear hatch, I was told several times that a replacement hatch is impossible. Both due to parts availability and functionally that they can't really do such a replacement at a body shop or service center. The body shop owner generally agreed with this, he says it's way more trouble to try and replace the rear hatch on a Model X than it is to effectively repair it while it's on the car. But either way, the body shop owner says he'll do the best he can, but I likely made a mistake taking delivery of this car knowing all these defects. Somewhere out there is someone who doesn't care about this type of stuff; they should have gotten the car hah.
