I can't answer most of your questions (I'm just another Airtable user), but I can answer some.
2-Factor Authentication is available for all plans and all users. In order to require 2FA, I believe you need to have single-sign-on, which is only available on business and enterprise scale plans.
The storage provided depends on the plan you have. You can see the attachment space allowed in the pricing page.
Airtable is a SaaS solution. There is no on-premise version of Airtable.
You may be interested in seeing the subprocessors that Airtable uses.
It sounds like security is a major concern of yours. The Enterprise Scale plan has an array of security features that are listed on the pricing page.