1. As you found, there is a specific limit for uploads to some Dropbox Business team plans of "1 billion API calls/month" for "data transport" . This is a limit on the number of upload API calls that can be made per team per month, for certain Dropbox Business team plans only. This only applies to upload calls, and not other types of calls such as download.
Additionally though, the Dropbox API does have a general rate limiting system that applies to all account types and calls, including downloads, though we don't have specific rate numbers documented for that.
Apps should be written to handle these rate limit responses automatically. Also note that not all responses with a 429 or 503 status code indicate explicit rate limiting, but in any case that you get a response with 429 or 503 status code the best practice is to retry the request, respecting the Retry-After header if given in the response, or using an exponential back-off, if not. I recommend referring to the error documentation and Error Handling Guide for more information.
And yes, these limits operate per-account. Note that the particular help center article you linked to is referring to a limit on traffic for shared links in particular, which is separate from the API call rate limits. The shared links traffic limit operates on the amount of traffic occurring on a particular account's links, while the API rate limiting system operates on the number of API calls per time window for an account connected to an app.
2. No, there isn't an iframe option available for the Chooser. You can find all of the available options on the Chooser documentation page that you linked to.
