Yes I've you have been busy. Any content relevant to the thread and useful for other users is greatly appreciated
For the product threads we're trying to keep the threads separate so posts in each thread stay relevant to that particular product. I'm not sure setting the forum (if possible) to automatically cross post anything in one thread to another would be a good idea as any posts relevant to one product, but not the others would get cross-posted and may cause confusion/problems. Also I believe Google de-ranks duplicated content on the same website so I don't think it would be a good way to boost hits.
I think the easiest solution would be to post links to the relevant content in other threads, or just simply cross-post post any useful content into the relevant threads yourself. I could always put a note in the first posts of each thread to make users aware of the other threads.
This is a limitation of the forum. The number of days to show active topics for is set by the individual user, there is no global setting for it. That said I've had a look though the back end code and found where it is set in the search result and upped it to 3 months.