What is the purpose of server_heartbeat_server when ft_active parameter is set?
If the following settings are for FT behavior:
ft_heartbeat                     = 3
ft_activation                    = 10

Are the following for ROUTES only?

server_heartbeat_server          = 5
server_timeout_server_connection = 18 

Resolution:
No, the server_heartbeat_server and server_timeout_server_connection are not for ROUTES only. These parameters are for a server-to-server connection.

The two servers can be connected either by a route, or as a fault-tolerant pair.

A backup server detects a failure of the primary in either of two ways:
•    Heartbeat Failure—The primary server sends heartbeat messages to the backup server to indicate that it is still operating. When a network failure stops the servers from communicating with each other, the backup server detects the interruption in the steady stream of heartbeats.
•    Connection Failure—The backup server can detect the failure of its TCP connection with the primary server. When the primary process terminates unexpectedly, the backup server detects the broken connection

server_heartbeat_server is used for identifying Connection Failure. If the the server parameters (server_heartbeat_server and server_timeout_server_connection) are not used (only the FT parameters) then  you'll rely on the OS to report TCP connection failure.