Basically you are doing it wrong. You should not assing other ethX interfaces than eth2 as it is WAN. If you want to use some LAN port as another WAN interface then take it out of the br-lan in luci and use appropriate lanX interface because with introduction of DSA (Distributed Switch Architecture) your lan ports behave as separate interfaces.
So for example use lan4.100 as your second WAN but be sure to take it out of br-lan to not make a loop in the network.
Also your ip link command is correct. There is still problem with multi interface switches with DSA driver in OpenWRT and for now all lan ports use the same ethX interface untill its fixed. And no you cannot assing lan ports to some specific eth interface. For now they all use one eth1 interface.