Site and Service Design

HI All,

   I got two Domain setup as account and Resources. Domain ABC - site and service setup. Now we got more sites comes under xyz domain.

1.     We have setup 2 x XYZ DC's in Site A (ABC domain)  and Site B (ABC Domain) and service DHCP for all new sites ( site E-F-G)   
2.       These two servers are in XYZ Site and services ( XYZ-SP1 & XYZ-SP2)  

Question:

1.     How do i add Ste A- G IP subnets in XYZ Domain site and services? 
2.     How do i setup browser selection?
3.     How do i clean this mess? remove all the ABC- DHCP servers?
4.     Do i have to add all the subnet in DNS revers Lookup zone? 

Issues:

•     Slow network access
•     Browser errors 
•     hardware maintennce

Attached the design. let me know how i solve above?

As