If you add a server it defaults to priority 1 so, yes the load will be spread over all priority 1 servers.
Retries are pretty much going away in the next beta. They seem kinda pointless in today's usenet. All active servers will get check and then the file will be assembled when the last chunk downloads or fails.
I haven't seen a modern server that really benefits from retries. I can think of one case, where post distribution is delayed on a server. I'm just not sure if that happens anymore.
If you have retries it'll do those before switching server which can add up if you let it do many retries and have multiple servers.
In 6.82, it'll check the other servers before retries kick in. 6.90B2 will toss bad servers, bad being "can't connect" into a bad server bucket so, it'll stop attempting to use those connections to the server. For example if you set 50 connections on a server that only supports 10, 40 of your connections will eventually get marked "bad" and it'll stop attempting to use them. This should prevent a dead server from stalling downloads.
B2 is on IRC. Dex and I are shooting to forum it this week.