Hi jake, you're pretty much on the mark, just a few clarifications.
1. Yes, only unique blocks will be written, same princle for dedupe applies to SILO or non-SILO no differnce there, so in other words wether or not the data is written to SILO Tape or not, only unique blocks are written to disk, of course if you seal the Store, now you're going to lay down a new footprint.
2. Data Aging will not do the 'purging' it will be Managed Disk Space thresholds set at the Library level which will control (primarily) when the purging can occur. This way regardless of retention (since you're going to have long term) you don't have to worry about how 'long' you'll keep the data on disk, its 'when' the library fills up, the system will self-manage that.
3. Check out this BOL section, explains a lot about the overall process: