As a quick test, not sure if this gives you what you want, I went to the external FS server and logged in as an internal user. I see the internal domains in the domain drop down. So I assume IWA to that box would work just fine.
All these domains are separate forests with trusts, so like you said this might amount to the same thing as one forest and many domains.
I pointed the internal user/computer to the FS instead of the FSP for the external domain. It worked as you said, I can get automatically logged in as an internal user directly to the external FS with IWA.
However, when this internal user is coming from the internet from home and wants to login, they'll need to login to the proxy FBA page. It seems that out of the box ADFS doesn't support this. Interestingly, I can login with just username, or username@domain, but only for external users. username@domain syntax doesn't work for the internal domains on the proxy, while IWA does work for internal users.
I am guessing that it will take some custom code on the ext FSP login page to get it to work, but I have no idea where to start at this point. If this can work, it would be a huge cost savings for the organization.