But what I describe is an alternative - you don't need to change anything on the existing router, it will just provide an uplink and address to the Mikrotik, and the Mikrotik will take over the rest.
The LAN hosts receive not only their own IP addresses but also the IP address of the default gateway from the existing router. So the only way to let the Mikrotik act as a bridge would be to let it do so only while the primary uplink would be fine, but once the primary uplink would fail, the Mikrotik would have to reconfigure itself into a router, start to impersonate the IP and MAC of the existing router to the LAN clients and deliver their traffic via the backup uplink, but at the same time monitor the primary uplink via the existing router and once it would start working again, reconfigure itself into a transparent bridge again. It is doable, but somehow it seems much more complex to me as compared to a regular dual-wan configuration.
Instead of writing novels, post /export hide-sensitive. Use find&replace in your favourite text editor to systematically replace all occurrences of each public IP address potentially identifying you by a distinctive pattern such as my.public.ip.1.