How can I connect to a specific machine with DFS?

You're seeing the intended behavior of the product. Accessing the DFS root using the domain's name, the "servera" name, or the "serverb" name will all result in the same behavior.

You might consider sharing the folder on "servera" and "serverb" a second time ("\servera\share-x", etc) and using that share name to access the folder with your replication software. Accessing the folder via this "secondary" share name won't cause the DFS client behavior you're seeing (since the "secondary" name of the folder isn't a DFS root or link target).

You could browse to admin shares of the physical disk instead of the actual share... i.e. \\server\d$\some\path\to\dfs\target instead of \\servera\target for replication purposes. Keep in mind... relying on 3rd party solutions for replication can present their own set of headaches.

I think I've run into every possible point of failure for DFS-R (and ntfrs before that), and have never had it successfully keep the peers in proper sync for more than a day or two. (before errors upon errors come up) I can completely understand your frustration and desire to switch to an alternative replication solution. You did make one incorrect assertion however... DFSR does keep backups of files in the dfsrprivate hidden folder... but running through all the logs to figure out which file is which and why it failed to replicate is a nightmare, and there is a maximum size to the "backups" before they are overwritten.

But at all points does the DFS target for servera contain the identical information that serverb does. DFS knows this, and offers you a valid alternative.