To answer your questions. It is true that traffic for an unpinned tenant is based off the proximity of the product (i.e. if it’s located in the EU, then Connect will route installations to your EU-declared app server in the descriptor). And if pinned, to the relevant pinned declared app server.
Now, unfortunately it is not possible to distinguish between installation routed based off proximity versus an installation routed based off the product being pinned. However, there has been talks about providing a new product API accessible by apps which enable app developers to obtain the residency of the tenant (whether it is pinned or not - and if pinned, to which location).
To answer #4, this is the intention of Data Residency migrations introduced in this post. It provides a mechanism for customers to trigger a migration for their apps, and subsequently a mechanism for app developers to listen to the migration lifecycle to move data between realms.
Now, an installation will continue to stay in the same realm unless an uninstallation happens. Then the routing rules will apply again. Subsequent upgrades and so forth will not redirect traffic to a new realm - even though it is compliant. This is explained here.
Hopefully that answers your questions! Please don’t be afraid to reach out if you have any further questions!