You can use the search function on the disucssion page (make sure you select the correct language first). It does work ok, but it's far from perfect. For older discussions you can also use Google. Queries of the form "duolingo username topic" used to be very effective (in fact, more effective than the internal search tool!) to find a specific discussion (i.e. if you know who participated in it). But it seems they stopped letting the site being crawled by Google, so this method may not work for more recent discussions.
The formal 'you' always uses the 3rd person plural conjugation of the verb. There is only one formal 'you'. There is no distinction between plural and singular forms. You use the same conjugation for addressing a group and an individual. (This does not hold true for the informal 'you', of course).