farcaller on DEV-10999 - Pro Micro - 3.3V/8MHz

I had a problem connecting this board via USB, so I wanted to verify that bootloader is ok and tried to reflash it (via arduino uno as ISP).

I suppose that the board would be 5V tolerant, so I plugged it to uno’s 3.3V and connected everything else as per usual ISP programming. Now, burning bootloader fails:

avrdude: verification error, first mismatch at byte 0x0000
     0xfe != 0xff
avrdude: verification error; content mismatch

Ok, I try powering it from 5V, failing again:

avrdude: verification error, first mismatch at byte 0x0000
     0xfe != 0xce
avrdude: verification error; content mismatch

Last try, I flash the 5V pro micro bootloader… and everything goes ok!

Now, I have a few questions.

The board is clearly 3.3V with LG33 voltage regulator and J1 being disconnected. Why cannot I flash it with 3.3V bootloader? The board, running with 5V bootloader is still underclocked, i.e. delay(1000) is actually a 2-second delay.

And a bigger issue – USB is still broken (is it burned? wrong programming?)