You doc looks great! Nice work! 
I’ll give you my ideas for your case 
If i had to, i will create a Users (or readers) table, where columns could be
Reader / Is this subscribed on mailing list? (like a checkbox) / others (like groups or other type of data)
This then could be moved in a view playable by users, where they can using buttons, add themselves or unsubscribe themselves from the mailing list
That would definitely needs readers to access to your doc on coda, that could be a cons for your user (need a new account, new interfaces to understand, ecc) or a PRO (if in that view of the users settings you create new settings, like “how many week mail?” or "What layout do you prefer for your email? or better "Do you want to receive all updates or just the “x-category” ones?
This could be an easy and interesting solution
Coda supports natively comments basically in every row or tables, but you’ll need your users to have an account and use them on coda (this depends on your choice from the above )
In this regard, i can suggest to create a formula to change the behaviour of users in coda using suggestion from the big @Krunal_Sheth
This allow you to become “Your Company Name Here” (without using another coda account made for the case), in any needed situation
That could be easily achieved if you choose to follow the “alternative” ruote, so to create your newsletter service directly on coda, here you are the master, you can add claps 
, hearts 
, dinosaur 
or whatever
This could be then relased in the Template Gallery and help every other future user 