I guess better late than never...
So we've got a failover IP, and a nice trick to be able to get the same WAN IP on both our routers. If you do stateless packet filtering, this is actually enough, and your redundant setup is already finished. However, if you do stateful packet filtering, the moment the failover IP moves to the backup router, your connection will be dropped because it doesn't have any knowledge about it in its connection tracking table.
In part 1 I went over the basics of failover. The problem with such a setup is that TCP connections will not survive a failover from ar0 to ar1, or vice versa. The main issue is that both routers have a different WAN IP, and in this case, seamless failover will never work.
Many corporate networks implement router redundancy for obvious reasons. Most of them are probably using very expensive hardware with proprietary protocols, but in fact it is possible to build such a setup for less than EUR100 and use it at home. Just get a couple of cheap routers that are well supported by LEDE/OpenWrt.
Some months ago, I ordered a Dommel SpeedConnect Plus connection. Since Dommel is using the network of the monopolist Belgacom in Belgium, I am forced to buy one specific type of VDSL2 router: Sagem F@st 3464. If you want to know why Belgacom forces this type of modem, you can find the report of the BIPT here, in English, Dutch or French.