Persistence profile - How will I use source address affinity based load balancing?

"Persistence" is about sticking to a common back-end pool. In your case, it is different pool member as the IP of the pool member is different. You can try to use Universal Persistence (UIE) with a specific hash value assigned to groups of pool members but it can get confusing. If you use the same back-end pool members, you can select "Match Across VS" within source IP based persistence.

Yes that's possible. Below an example that you may need to adapt to fit your needs :

when HTTP_RESPONSE {
    persist add uie $clientip
}
when HTTP_REQUEST {
    set clientip ""
    if { [HTTP::header exists "X-Forwarded-For"] } {
        set clientip [HTTP::header "X-Forwarded-For"]
    } else {
        set clientip [IP::client_addr]
    }
    persist uie $clientip
}

Yes that's possible. Below an example that you may need to adapt to fit your needs :

when HTTP_RESPONSE {
    persist add uie $clientip
}
when HTTP_REQUEST {
    set clientip ""
    if { [HTTP::header exists "X-Forwarded-For"] } {
        set clientip [HTTP::header "X-Forwarded-For"]
    } else {
        set clientip [IP::client_addr]
    }
    persist uie $clientip
}

Don't worry, it's a common scenario for me.

Just pay attention that the upstream proxy insert the "X-Forwarded-For" header, or maybe another one "True-Client-IP" or even "X-Client-IP".

If you are just nating using a firewall, I think that the header is not inserted.

I add this peace of code in the example :

set clientip [IP::client_addr]

because you may have situation where there is no "X-Forwarded-For" in the request, so that we can persist also in this scenario.