To partially answer my own question, EasyTether does not support ICMP(PING) which is why the interface is shown as Offline. In, mwan3, I switched it to use httping with hostname google.com which does bring it Online initially but then a few seconds later it goes Offline again.
12: tap-easytether: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc fq_codel state UNKNOWN group default qlen 1000
link/ether 02:00:54:74:68:72 brd ff:ff:ff:ff:ff:ff
inet 192.168.117.2/24 brd 192.168.117.255 scope global tap-easytether
valid_lft forever preferred_lft forever