doesn't work as expected about wanfailoverscript HOT 4 OPEN

Strange, this script is running in production on multiple hosts for years.

Can you please test with the previous version?

The script specifically creates static routes to the target hosts, so when 
switching to the backup link, those target hosts won't be pingable as long as 
the primary link is down. 

I do not understand your second remark about MAX_LATENCY. This is how long PING 
will wait untill the response is considered timed-out. 

The MAX_LATENCY value is incorrectly stated in miliseconds but it actually 
works in seconds. So 1000 seconds is a bit too much. This must be changed and 
is a bug.

Checking a .target host that's visible only from primary link is very narrow 
condition in my opinion Maybe simply ping -I $PRIMARY_INT would be more useful, 
also maybe some other other checks like dig -b $PRIMARY_IP ....
Hping utility could be perfect for a comprehensive check!

This script does not and will never rely on non-standard third-party tools. 
This is by design.