1/ Does gallery resizing work inside an iFrame?
Yes. As long as the dimensions of the iframe and all parent containers have heights expressed as percentages via CSS.
2/ How does a gallery appear on an iPad/iPhone if it is embedded inside an iFrame? (Do I need to build a second site to better suit iOS devices?)
A gallery will look the same when embedded in an iframe as it will when embedded using the recommended embedding code with the exceptions that the 'Expand Gallery' button is disabled and the 'Back Button' will not work. Please see here for details.
3/ Is there the possibility to use a top-location parameter, to ensure that an iFrame gallery cannot be viewed without its 'frame'. (This is really important to avoid anonymous galleries).
Yes. You could add JavaScript code (to check whether or not the gallery is displayed inside an iframe) to the 'index.html' page of a gallery. If the gallery is not displayed inside an iframe, then you could redirect the user's browser to the page you wish the user to see. Knowledge of JavaScript would be required.
4/ Can thumbnails be set to 'autohide'?
You can set showLargeThumbs="FALSE" to initially hide the thumbnails and set showthumbsButton="TRUE" to allow the user to display or hide the thumbnails via the Thumbnails Button on the Button Bar.
5/ Is IPTC data that is not used for display, but is embedded in the images, available to web-crawlers? (I'm thinking specifically of Keywords)
Google may be able to index the image data stored in the gallery's XML file (try a web search to find out more about Google's web-crawling algorithms) but it looks like it would not be able to crawl through the meta-data stored within the images themselves. Please see this Google forum thread.
6/ Can I place a copyright text anywhere in the stage?
You could use the Gallery Title text field to add copyright text to your gallery. The Gallery Title can be positioned OVERLAY, TOP, ABOVE_THUMBS (or NONE). Please see the General Options section of the Config Options page for details.
You could also add copyright text to either the Title or Caption text fields of each individual image.
Your first three queries apply equally to Juicebox-Lite (the free version) as they do to Juicebox-Pro so you could try using Juicebox-Lite (which can be downloaded from this web page) before purchasing Juicebox-Pro.