You are right, it is impossible to force it to 40MHz. Even setting "option htmode HT40+" in etc/config/wireless does not make wireless use two channels.
MT7620 support is broken and I believe 5GHz is only way to use this router under LEDE/OpenWRT (not working under 18.06, unfortunately).
Regarding the bug: something has been done in 18.06 as router will survive "full tx queue" but will lock at "ieee80211 phy0: rt2x00lib_rxdone_read_signal: Warning - Frame received with unrecognized signal, mode=0x0001, signal=0x010d, type=4" instead :(
I just got it to lock by copying a file from NAS. Checked through SSH, CPU was hovering around 12% while copying so obviously, CPU is not the issue.
I am considering switching back to factory firmware.