Ksger STM32 hot air station - seppuku? Help with repair

[takio]

Hi all, long time viewer, first post.

So I ordered a KSGER hot air station from the Aliexpress "official" store:
https://www.aliexpress.com/item/33037184838.html?spm=2114.12010612.8148356.3.16862ed8YcDkNp

I'm not a professional by any stretch, and my work volume doesn't justify a costly station.
But yes, I got what I paid for, didn't I?

When it arrived, of course I didn't turn it on, I took it apart, for inspection and safety purposes. Didn't find much to worry about other than a sloppy inductor bungled onto the control board. Reflowed it straight, grounded the metal case, soldered the triac heatsink to the pad it was floating above, and turned the thing on.

When powered on, at first everything looked fine, but when closing the circuit on the rear button that engages the wand fan, the power supply got shorted and the whole thing shut down. Disengaging said button allowed the power supply to come back and the unit to power back on. This is also signaled by a red LED on the power supply board, not visible unless the case is opened.
The "wand", containing both fan and heating element, was not connected to the unit. Connecting it made no difference, so its absence was not apparently causing the issue. Disconnecting everything from the control board, apart from the power and the fan switch, made no difference. So that switch is causing a short back to the power supply, via the control board.
A few tries later, it all went belly up: when toggling the switch, there were sparks and magic smoke signals emerged from the vicinity of U3, a step-down converter near the pins that receive electric pixies from the power supply.

Without much else to lose, I decided to go all in on this, armed with my vast lack of knowledge.
Lots of stuff out there about the infamous T12 KSGER soldering station, but not much about this hot air station.
The control unit, however, also based around the STM32 chippy, seems to be largely similar to that of the soldering kit.
Based on what I could gather from various posts around here:

The board seems to follow pretty much the same as found on the schematic posted here: https://www.eevblog.com/forum/reviews/t12-stm32-v2-1s-soldering-station-controller-schematic-etc/

Observations and changes near the control board power input:
> U3, marked JWBHJ 9CA7L, blew up and was reading a short across a number of pins, discarded and replaced with TI LMR16006YDDCT DC-DC step-down;
> L1/L2 reading a short, replaced with chunkier 4.7µH ferrite core inductor (larger package than supplied but still smaller than what is on board photo on the product page);
> L1/L2 inductor pads read a short, even with no inductor mounted on them;
> VCC and GND pins to the display read a short, 0.5 Ohm;
> VCC and GND pins (unpopulated, programmer header?) above U1 (the STM32) read a short, 0.2 Ohm;

Observations near the connector for the fan switch:
> Diode D4, marked RS1M (fast recovery rectifier) reads ok;
> Q1, marked ARDV17, reads a short across all connectors: readings of 0.2 / 0.5 / 0.8 Ohm across pairs;
> Resistor R15, marked 472, reads 4.7k when off the board, and reads 0.5 Ohm when soldered;
> Unmarked component below R15: similar to a through-hole small diode package, but I think it's something else, no idea, maybe some kind of fuse. On the product photos, it's not present on the board.

I couldn't find the exact reference for U3, and of course I might have bungled everything up by replacing it with a different pinout or one that requires different configuration values.

I grounded the case by connecting the ground pin on the power socket to a crimp connector screw-mounted to the case back panel, and sanding down all the case edges to make sure each part is grounded, including the encoder housing and wand connector. A known fix for the soldering station, but could it have been a bad idea here?

Also may have inflicted thermal damage to the power supply triac when soldering down its heatsink pad, but thought it should be there for a reason, and these little buggers are usually fairly rugged. Still...

As is, the power supply powers on and yields 24V at the output, but the control board is dead and reads shorts all over the shop, the display doesn't come on, and the back switch (that goes to the control board) still causes the power supply to shut down.

I am expecting warranty service, at least a replacement board, and will submit for a refund if the warranty is not honored. But I'd much rather learn something along the way by getting this unit working again, not to mention avoiding the waste.

Some pics below to illustrate the key points mentioned.

I appreciate any help

Thank you all