DNS changes needed for outsourced website

[sintral]

OK, I'm sure I'm making more of this than what it is.

I have two domains: domain1.com and domain2.com. Domain1.com is
registered through Register.com and uses it's 3 name servers. Here
onsite we have a Bind8 DNS server that handles several hosts for
domain1.com such as ftp, mail, www, smtp, etc. Domain2.com is
registered through eNomCentral and uses 3 name servers from Earthlink
where the website and a few user email addresses are hosted.

www.domain2.com and www.domain1.com both need to now point to the IP
address of our new website, which is outsourced. Basically, there was
a company merger and both domains should point to the same site.

The other hosts on domain1.com still need to resolve to our onsite
office for purposes of mail, ftp, etc, so I've modified the Bind entry
for www.domain1.com to point the public IP of the new website instead
of the private internal IP that it used to point to. The combination
of that DNS record and our Cisco PIX firewall rule of pointing all
port 80 traffic to that same internal IP worked for both internal and
external users.

I have several questions so far:

1. How do the name servers at Register.com know to send all traffic
directed to domain1.com to my public IP? This works currently, but I
can't figure out how. Best I can tell they are using their own
nameservers and there is no reference to my internal Bind8 server.

2. Is the change that I have made to the internal DNS server the best
way to do this? The before and after looked like this:

domain1.com.            IN              NS         ns1.domain1.com ;
This is my local DNS server
ns1.domain1.com.      IN              A 	      10.6.18.2 ; internal
private IP of DNS server
ftp.domain1.com.       IN              A           10.6.18.10 ;
internal private IP for FTP server
www.domain1.com.    IN              A           10.6.18.179 ; private
IP of internal webserver (before)
www.domain1.com.    IN              A           200.150.200.150 ;
public IP of outsourced site (after)

3. I have conduits and static routes setup in my PIX that still point
to the old webserver. Apparently the new website will point to the old
server for certain pages that haven't been migrated over yet. I
realize this is off topic, but are there any considerations that I
should know about with that setup?

4. Will this setup allow internal and external users to access the new
website at the public IP? What I mean is, is there anyway that the
firewall can/will interfere with what I'm trying to do with DNS?
Basically, I want all traffic from domain1.com to hit my firewall and
be directed to one of several hosts EXCEPT host 'www', which should be
redirected to the new IP.