Summing up all the exploratory endeavors, the answers are:
- The ‘band’ setting sets the maximum supported WiFi 802.11 standard, with all standards below it always supported. And if the interface doesn’t support set band, it fails with the ‘no supported channels’ message. If the ‘band’ setting is unset, the interface will operate as if the ‘band’ setting was set to the most recent 802.11 standard that is also supported by the interface. All in all, leaving it unset seems to be the best option with RouterOS 7.13, especially in mixed ac/ax CAPsMAN-controlled networks.
1.1. No, you cannot turn off 802.11b.
- The rate configuration went out to get the milk, and didn’t come back in wifi-qcom-ac. (i.e. it is currently unavailable with new wifi drivers)
Thanks to @whatever, @infabo and @mkx for helping solve this case.