In regards of SEO, you should just stick to the fundamentals as if you were making a single site. However, if I am understanding it correctly, you are trying to setup a multisite. Refer to this link for very important info regarding SEO for multisites and multiple TLDs websites
In regards of your breadcrumb. You can set up a breadcrumb with each domain very easily. However, one thing that you should remember is that a breadcrumb and a wayfinder are two different things. A breadcrumb is not a history. Its purpose is to show a user how to get to that page from home. So basically it can be done statically. Here's an example of the logic I would use.
HOME >> link 1 >> Link2 >> link 3 >> YOU ARE HERE.
It takes some good mapping and site architecture to get it right. If the domains used are all hosted in the same server and you are using a multiple TLD site, you can use some dynamics to this code and post back to the server your location and have it return the name of your location or even from a database. But this is all unnecessary action on the server.
A wayfinder, is history based. This system posts back to the server the pages visited so that the visitor knows how he got there. In my opinion, this is pointless. You should only provide the visitor with a way back, a way out, and way to begin. That's it. You don't need to show a visitor all 300 pages he clicked to get to where he is now.