If I implement the pagination as www.domain.com/content?page=1,
...page=2, etc, will SEO pick up the GET params and treat them as
non-duplicates and crawl them appropriately?
Google crawler will be able to identify that the content changes based on the 'page' parameter. To be sure, you can use the parameter handling tool available at 'Google Webmasters'. See this page from Webmaster's Help for details on how to use it.
since this content is continuously being modified, the max # of pages
can and will change, therefore do I need to add all the possible page
numbers to sitemap.xml or will linking them from the first page be
sufficient to be picked up by the crawlers?
I recommend that you add all the URL to your sitemap, if possible. This will help the crawler better understand your site.
This is what I would suggest to you :
Use autoscroll but do not hide the pagination links. But instead of automatically loading new content on scrolling down, add a button which says 'Click to show more items' or a checkbox saying 'Auto Scroll'. This is necessary because you said that there will be many items and if all of them are loaded with auto scroll, the browser will become too slow. By activating scroll on button click, you will allow the user to navigate to even the last page in the paginated list which might not be possible with auto scroll due to the huge size of the document. And with the paginated links, Google will crawl your site properly.
And if the content in the paginated list changes rapidly, ( assuming that new items will be added on the first page ) then the Google crawler may not be able to catch all updates, especially if your site is new. So, I suggest that you add a button which sorts the items based on oldest first. The advantage is that the content on all pages except the last one will stay constant and this will help the crawler crawl and index your site properly. And to avoid duplication of contents in the two paginated lists ( oldest first and newest first ), block the 'newest first' pages ( except the first few pages ) using appropriate declarations in your robots.txt.