robrecord

To potentially get rid of garbage you can save your .afpub under a different name. The success depends on the document, its content and its history. For instance my current .afpub just reduced from 30 to 25 MB – whereas others report no or minimal success. Worth a try at least.
I can't answer your 3 questions but as far I remember there are already quite a few threads about .afpub file size with .afdesigner or .afphoto links after the v331 + v337 update which brought the truly linked resources.

The pre-order was completed without a second thought. You have no idea how excited I am for this release.
There may have been dancing.

@A_B_C @hperticarati Nested symbols are indeed currently possible, but they are very buggy indeed! The nesting often breaks one or both symbols (orange line in Layers panel becomes dashed) or the symbol simply vanishes (at which point Undo will not bring it back).
 
Nested symbols are a key feature for me in my work, and at present it is a real struggle to use them

Yes, that is precisely what I had in mind... afyer having all my symbols unlinked from master a bunch of times, I’m kinda stucked in the decision of re importing all symbols (which is going to be a lot of work, considering that some of them should have been updated with changes I made directly in their instances), or switching to another tool tha handles symbols with more stability. 
 
So maybe, instead of asking for “nested symbols”, the key words should be “a more stable symbol workflow”.

You cannot re-sync a previously un-synced attribute of an instance of a symbol. The built-in help & the video tutorial are not very clear about this. The Symbols help topic does say this:
... but it also says this:
The "until synchronization is enabled again" part implies turning sync back on for a symbol instance would re-sync all its attributes but obviously that is not how it works.
 
IOW, while synchronization is disabled on any specific symbol instance, a change to any of its attributes is permanent & cannot be undone by reenabling synchronization. As you mentioned, the only way to do that is to replace that instance with a new one from the Symbols panel.
 
It might be useful to have a forth "Resync" button in the panel to do that....