Did you find a way to fix the broken “push-push” spring retainer mechanism, or have to RMA the board back to Mouser?
Alas, I have not found a good solution to this issue.
When the board was “naked”, simply tied to its power supply, I could easily put some adhesive tape and hold the SD card inside without too much troubles. I now have it inside a PC case, and it’s very difficult to hold the SD card inside with the same adhesive tape. The board’s backplate and its frame design is simply not compatible with such trick. The adhesive tape is not retaining the SD card enough for it to work.
I changed my SSD recently and so I can’t use the solution that you have found (hold the SD card manually during the boot-up procedure with an existing install on NVME).
I looked at the BOM and found the corresponding SD card connector from Molex at Mouser, in stock.
So I have two solutions now:
 - replace myself this SD connector: it’s doable as everything is soldered on one side of the PCB, and there’s not a lot of components around that I can damage. But it voids the warranty then.
 - go for the RMA as you mentioned.
It’s an unpleasant situation anyhow as I waited 6 months to get this board and if I take the RMA path, I don’t know when I’ll receive back a new board.
On Molex web site there’s a 3d model that I could open with FreeCAD to understand how the spring mechanism is working. There’s a small “U lever” that follows a path molded in a black plastic part. And on my case this lever has jumped outside this molded path. So the card is simply not retained inside through this lever. The spring is ejecting the SD card, always. And I haven’t found a way, even with a small pincer, to replace this lever inside its path. Anyhow, I doubt that this would work for long. I found the overall mechanical principle to be too weak, in my opinion. It’s not suprising that you had this issue too. And some others will probably follow.
All in all, I think I’ll go now for a RMA and hope to not face again the same issue.
I’ll buy a SD card adapter like this one:
… to plug once this adapter to the SD card connector and not touch the original connector anymore. If something breaks, it will be at adapter level, I hope.