This isn't my area of specialty, but I can probably answer the basics.

The Stinger missiles thing is fairly straight forward - the Tunney/Clark Amendment was repealed in 1985. The Stingers were delivered in 1986. Here is a contemporary article on the repeal:

http://www.nytimes.com/1985/08/10/opinion/l-good-riddance-to-the-clark-amendment-182085.html?mcubz=1

As to why the US was unwilling to get more involved in the Angolan Civil War:

• Vietnam syndrome. Get involved in a messy civil war in some country Americans don't know much about to stop the communists? Sounds a bit too familiar.

• The desire to limit executive authority/the CIA. My reading of the Clark Amendment and similar acts from the era was that legislators weren't necessarily willing to let 'the bad guys' win... they just didnt' think the president or CIA should have a free hand. It wasn't that long after Watergate etc.

• The US didn't necessarily want to be seen to be directly supporting South Africa due to apartheid being a point of contention internationally.

• The US may have thought escalation on their part might just encourage Soviet/Cuban escalation. The Cubans after all pointed to South African intervention in explaining their own deployments.

• Desire to maintain detente. The USSR was seen as being behind the whole thing, but in the spirit of detente diplomacy was the preferred solution. A heavier response might have spoiled that, and it could have been spun as the fault of the US.

You may have previously seen this memo: http://nsarchive.gwu.edu/NSAEBB/NSAEBB67/gleijeses4.pdf But it covers items such as hesistance to get involved while South Africa stood to benefit, and attempts to side step controls on aid for the US, by having 3rd parties do it for them.