Domain – Purposefully break trust relationship with Windows Domain

I don't think your domain controller is ignoring anything - this is all a function of the client and which queries it chooses to send along to the domain controller. Parts of your question are a tad confusing but you should just add one domain suffix to the DNS config of each PC that matches the local forest and another domain suffix that matches the remote forest. List the local forest first.

The strongest reason to not go with a single domain (and forest) is if you absolutely have to have separate admins in each forest. That's the security boundary. If the same folks are going to have the full set of keys, then make it easy on them. To be clear, I'm not talking about delegation for certain tasks - this is the group of people who are going to be enterprise admins.

This doesn't change my recommendations much, because you can delegate things out as needed, as I said above. If part of the org has local admins that need to be able to edit the GPO that is assigned to their OU, you can give that to them, as an example. However, if they need total control over their part of the domain, to the point that they need to be able to lock you out of it, then you need separate forests, and might have a tough time sharing exchange. So, since you're sharing "security and exchange", it sounds like a single domain is still the right way to go.