From the BGP neighbor status output, the key issue is that BGP is stuck in the "Idle" state, meaning the FortiGate is unable to establish a BGP session with its peer 100.65.4.1 (Remote AS 65300).
The output also shows:
# "Not directly connected EBGP" # This means the BGP peer is not on the same subnet, requiring multihop BGP.
# "Update source is Loopback" # Since a loopback interface is used, FortiGate must be configured to allow BGP neighbors over multiple hops.
To resolve this issue, the administrator must enable ebgp-enforce-multihop, which allows BGP sessions to be established even when the neighbors are not directly connected.

In the Hub2Hub peering, ebgp-enforce-multihop is required because the EBGP session between regions must support peering beyond a directly connected single-hop expectation in the ADVPN design.
attribute-unchanged next-hop is required so the original BGP next hop is preserved across regions. This allows the remote side to learn the actual tunnel endpoint information needed to build dynamic ADVPN shortcut paths between overlays.

In this scenario, the corporate network and the new remote office network need to communicate over the Internet, which requires a secure and dynamic routing method. Since both networks are using OSPF (Open Shortest Path First) as the routing protocol, the best approach is to establish an OSPF over IPsec VPN to ensure secure and dynamic route propagation.
OSPF is already running on the corporate network, and extending it over an IPsec tunnel allows dynamic route exchange between the corporate FortiGate and the remote office FortiGate. IPsec provides encryption for traffic over the Internet, ensuring secure communication. OSPF over IPsec eliminates the need for manual static routes, allowing automatic route updates if networks change.
The new remote office's 192.168.1.0/24 subnet will be advertised dynamically to the corporate network without additional configuration.

From the OSPF status output, the key information is:
"This router is an ASBR" - This means the FortiGate is acting as an Autonomous System Boundary Router (ASBR).
An ASBR is responsible for injecting external routing information into OSPF from another routing protocol (such as BGP, static routes, or connected networks).

ICMP is a connectionless protocol. In an FGSP cluster, connectionless sessions such as ping are synchronized only when session-pickup-connectionless is enabled. Because FortiGate_B shows no ICMP session entries, those ICMP sessions are not being synchronized to it, which causes the observed output.