Caveat: I am not an admin and have no control over this site. That said, here are my answers:
First question: it seems that the Requests page is a list limited to, say, 50 songs. When a new request is submitted, the oldest request gets dumped. Most requests die by this mechanism, without having been tabbed. I don't think that a song in the request list disappears because a tab has been submitted. New request come in too fast. They disappear because they have been bumped from the list by newer requests. Again, most requests are not tabbed. (I want a way to send private messages so that I can let a person know I tabbed a song they requested, and ask that they rate it. But for that, we would have to give our email addresses to Big Bass Tabs, which may not be a good idea.)
Second Question: I agree that a page for sheet music, or perhaps links to sheet music would be helpful. I usually try to have a look at the sheet music for a song when I want to tab it. (No - I can't sight read, but I can figure out the notes.) You can often see the first page of sheet music (e.g. musicnotes.com) which gives you huge help in starting a tab (time signature, key or at least relative intervals, length of measures…
I think there would probably be a copyright issue with posting sheet music though, so maybe not possible.