Using pins D9 & D10 on the MDBT42Q

You really did mean an app (not going down that route)

You mean you never install any apps on your phone? Honestly the NRF Connect app is harmless, ad-free, and very useful when developing with Bluetooth LE.

why do we need to restart the "softdevice" to be able to use the poke32

poke32 works totally fine with the softdevice running on most addresses, but there's a certain level of memory protection added while the softdevice (bluetooth stack) is running - it doesn't want you to be able to modify internal registers like UICR.

So what we do is disable the Bluetooth stack, write them, and restart it. You also have to fully reset the device after setting the registers in order for the pins to work.

But you only ever have to do this once for each device.