How about IF register?

//disable timer
  LETIMER_Enable(LETIMER0, false);
  //clear interrupt
  LETIMER_IntClear(LETIMER0, LETIMER_IF_UF);

I don't have enough time to read and understand your code.

As to TIMER4, I am wondering if your code handle SFRPAGE correctly.

	SFRPAGE = 0x0F;

I (possibly erroneously) setup my SPI pins as the SWD pins and uploaded the program.

I guess your code make SPI to use PA1/PA2, right?

I managed to get the device to a flashable state by using:

commander recover ....

usually this is a good way to recover this situation.

What is the correct bootloader for the BGM210P?

You could use the simplicity studio to generate the 2nd stage bootloader for xG21 (read UG266).

@NCF,

but I'm concerned first on how to flash the booloader itself.

the gecko bootloader is very powerful and support bootloader update (plus application). You could take a look at UG266. 

Yes, if you use the DCDC to power the IOVDD, there is some limitation to program the target with SWD. also take a look at this KBA.

https://www.silabs.com/community/mcu/32-bit/knowledge-base.entry.html/2017/07/20/debug_connections--9AvG

I guess I'll go with the SWD + external IOVDD approach.

This will make things easy if you are ok with it.

In the datasheet (you have a link for it), you could take a look at table Table 8.3. Alternate Functionality Overview.

take the EM4WU as example, you could get which pin support this like below picture showed.

As Gene said, all pins support EM1/2/3 wakeup, the EM4 pin may support wakeup/interrupt with level sensitive IIRC (refer section  Level Interrupt Generation of reference manual).

I (possibly erroneously) setup my SPI pins as the SWD pins and uploaded the program.

I guess your code make SPI to use PA1/PA2, right?

I managed to get the device to a flashable state by using:

commander recover ....

usually this is a good way to recover this situation.

What is the correct bootloader for the BGM210P?

You could use the simplicity studio to generate the 2nd stage bootloader for xG21 (read UG266).

Check if this helps.

https://www.silabs.com/community/mcu/8-bit/knowledge-base.entry.html/2004/04/21/in-line_assemblyin-BBF4

Thanks for sharing. I think this help others a lot!

I cross link the other post/thread.