Your best bet to find the answers are to search the Delta and United forums, particularly if you like to do things yourself....
Most of your questions have been answered ad nauseum.
For example, the Singapore Airlines and Delta partnership is ending. See http://www.flyertalk.com/forum/delta...ymiles-73.html.
Rules for stopovers and connections on United award tickets:
http://www.flyertalk.com/forum/unite...ard-rules.html
Round-the-world awards and Star Alliance (which united is a part of):
http://www.flyertalk.com/forum/star-...questions.html
Comparison of Delta/United RTW awards:
http://www.flyertalk.com/forum/delta...rld-delta.html

As to your individual questions about whether certain flight combos will be "allowed", there isn't going to be any place that lists every possible city pair and what options are 'allowed.'