counterpoint

Thanks very much. Sorry for the slow response - I'm only allowed one post per 24 hours. I seem to have a solution. As mentioned above, I took the image from https://www.armbian.com/odroid-hc1/
 
To be more specific, I used the one with the large image at the right hand side of the page, just below the picture of an HC1. Looking more carefully, it uses kernel 4.14.y. Not being knowledgeable about kernel versions, I assumed that the highlighted image was the best to use. However, I later looked down the page, and tried using the download link https://dl.armbian.com/odroidxu4/Buster_current_minimal which uses kernel 5.4. Why would people prefer an old kernel? Anyway, everything seems to work as I would expect with no need for selecting legacy elements or anything unusual.
 
With the other image, I was not able to find a package called iptables-legacy, only one called iptables. Reinstalling it and rebooting didn't make a difference.
 
In case they are still of interest, the answers to your questions are:
root@backup:~# find /lib/modules/|fgrep -i tables /lib/modules/4.14.187-odroidxu4/kernel/net/ipv6/netfilter/nf_tables_ipv6.ko /lib/modules/4.14.187-odroidxu4/kernel/net/ipv6/netfilter/ip6_tables.ko /lib/modules/4.14.187-odroidxu4/kernel/net/ipv4/netfilter/nf_tables_ipv4.ko /lib/modules/4.14.187-odroidxu4/kernel/net/ipv4/netfilter/ip_tables.ko /lib/modules/4.14.187-odroidxu4/kernel/net/ipv4/netfilter/arp_tables.ko /lib/modules/4.14.187-odroidxu4/kernel/net/ipv4/netfilter/nf_tables_arp.ko /lib/modules/4.14.187-odroidxu4/kernel/net/netfilter/nf_tables_netdev.ko /lib/modules/4.14.187-odroidxu4/kernel/net/netfilter/nf_tables.ko /lib/modules/4.14.187-odroidxu4/kernel/net/netfilter/nf_tables_inet.ko /lib/modules/4.14.187-odroidxu4/kernel/net/bridge/netfilter/ebtables.ko /lib/modules/4.14.187-odroidxu4/kernel/net/bridge/netfilter/nf_tables_bridge.ko root@backup:~# armbianmonitor -u System diagnosis information will now be uploaded to http://ix.io/2rqQ