>>> Q2: Why such route-map is applied on Border-only switches? Right now if Fusion1 fails, B1 has no exit path which would result in a negative impact for SDA endpoints.

>>> Q3: Application of this route-map is manual or automated? I know this route-map is also used under LISP routing process but I didn't find any documentation about applying it under BGP routing process.

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Q2: From my research, it looks like this route-map "deny_0.0.0.0" is useful under BGP routing process and applied to eBGP neighbors, only in the case of Internal-only Borders since this Border type should not advertise a default route into the SDA Fabric.

Removing the application of the route-map doesn't break anything. It brings the default route redundancy on B1 and B2 nodes.

Q3: I believe it was manually applied but cannot confirm.

Still investigating on the other questions...