Load Balancing Default Pool

depends on how you configure it, if it is just basic setup with only one default pool and nothing else (irules, http profile sorry server, ...) and no members are available in that pool then it drops it.

This depends on the default pool's behavior. The "Action on service down" behavior dictates the action, and can vary from allowing established conections through to rejecting or dropping the packets. The only instance where an alternate pool would be selected would be with an iRule, or there are options within HTTP profiles for fallback IPs.