As I mentioned, we failed to set this mode in accordance with Keycloak documentation, so we have chosen our own path and the performance is good for our case.
Regarding your second question, Keycloak does not guarantee zero-downtime upgrades regardless of the architecture you are using. 11.0.3 to 12.0.1 Upgrade fails - #5 by dasniko. E.g. we cannot upgrade from 11.x to 12.x version without losing all authenticated sessions. We are performing zero-downtime upgrades while staying on the 11.x version. We drain traffic from one region, upgrade that region, swap traffic to the upgraded region and upgrade the second region. Each region has a cluster of 3 Keycloak nodes. Cross-region infinispan caches replication is enabled by setting this option: CACHE_OWNERS_COUNT=4, so at least one copy of the cache stores in another region.