I have emailed a contact at Apple who is going to get back to me re: Q2.
re: Q1 I did some quick testing which revealed the following -
The example RSS code I posted for multiple categories is correct. As I mentioned previously, however, iTunes U only takes the first instance of the itunes:category element in each feed item and all other instances are ignored. For that reason Apple advise that you select the single most relevant category for each of your feed items.
In our case we're creating our RSS feeds programmatically in SQL and the iTunes U categories are also listed using the standard RSS <category> element. In addition to helping feed aggregators categorise content it offers the user the ability to filter the RSS feed by category in some feed readers - see example screen shot in IE below. For that reason we are going list multiple categories for each item in our feeds and ensure that the most relevant category (the one picked up by iTunes U) is listed first.
It is also worth noting that the category that is applied to your collection will be the dominant category at item level in your RSS feed. For example, if you have three items in your RSS feed, one is Social Science > Law, and two are Business > Economics, the category applied to your collection would be Business > Economics. The category for your collection shows in the category column of your collections page in PSM (see screen shot below). The category applied to your collection may change if new items are added and they have different category, it will all depend on which category is dominant. If no categories are applied in your RSS feed it will be listed in PSM using the default category 'iTunes U'.
I'm afraid I long ago gave up trying to understand how iTunes U categories work and how category info should be represented in the RSS feed. I'm not even sure if what I have written above is correct not least because iTunes U has changed quite a lot over the past couple of years. The links to the old admin guide, for instance, no longer work. In the end I gave up because most of our content is multidisciplinary and would not fit under a single category.
Your message has piqued my interest, however, and I found the following links which might be of interest -