Lync Server 2013 in large organizations edge pool and external login

I found this technet reference but have some questions on it.

If I point the all external DNS record to first site A, the sip federation still work on Site B?

If our user is home on Site B , our user can through Site A reserver proxy to re-directly to Site B login?

If I point the all external DNS record to first site A, Site B  user still using Site B pool but both have separate different DNS  record for egde pool and different public certificate right ?

https://technet.microsoft.com/en-us/library/gg398797(v=ocs.15).aspx

Thanks.