The answer to your first question is tricky. You can use a summary address for a site that has multiple subnets. You will need to monitor this configuration and make sure everything is working as expected. While this method will work, it's officially unsupported by Microsoft. There are a few issues reported with SCCM not working correctly using supernetting. I would watch your logon traffic and make sure clients are being assigned to the correct site.
When you access the properties of the defaultipsitelink,, the schedule indicates whether or not replication is allowed at those times, but not the frequency. This would apply to all replication that occurs across that site link. When you look at the NTDS site settings properties, this controls how often domain controllers in that site attempt to replicate. Once an hour is the default. This only applies to the domain controllers in that site, as it is a per site setting. If the site settings say to replicate twice an hour, but the defaultipsitelink settings are set to no replication for that hour, then they will not replicate.
Hope this answers your questions, if not, let me know.
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