welcome to the community! 
Cross domain sharing is prohibited when sharing with a gmail user - #9 by Vlad_M
there is a known bug that some email addresses (newly created, gmail etc) get flagged and arent able to be shared. For that, please reach out to coda support directly and they can unflag support@coda.io or open a chat by selecting the question mark in the bottom-right corner of your Coda doc.).
sorry to hear that youve spent hours on coda and it has not met your expectations.
in efforts to understand where you’re coming from…is this the “marketing site” you are referring to ? Pricing, pricing plans - Coda
and where you’ve said “invite viewers, always free.”, its in reference to this?
to be frank, from my understanding what is stated is what is promised. One can invite users and it is free to do so.
Its just the “how” that is not explicitly stated.
I assume you’re a dev/technical person and that you’ve interpreted it as via API etc ?
Please note for common folk (non techie) and overall general public, it just means you can manually do it (from inside the product/Web UI i.e. Share option in a coda doc).
I feel that maybe running into the bug described above has compounded on the validity of said statement.
In terms of their API access- i’ve always thought and seen that it is general industry standard/practice for SaaS to charge for access to their more niche/specialized APIs.
Which is fair enough. Companies need to be able to generate income somewhere so they can maintain and develop their software. And if you dont agree with that, thats why open-source software also exists.
Its important to keep in mind, Coda has to keep their lights on as well. They have their team to feed and a billion dollar loan to pay back 
cheers!
Mel