Most airline procedures prohibit the use of reverse thrust when one (or more0)engines have failed. Plus, the effectiveness of only one reverse thrust would not have slowed the aircraft much. It is pilot discretion on flaps, and you do not have to perform every landing with full flaps. Depending on the wind, the pilot may have elected to go with less than full flaps. But since the plane took off and suffered an engine failure, you can bet that the aircraft's weight factored into a faster than normal landing speed and may also be why the landing was completed with less than full flaps.
While I've only flown Cessna's (152 and 172) I can say that most of my landings were completed with less than full flaps.
Do you have a list of those airlines that prohibit that? Because the last two I've flown for didn't have that restriction. And it is MOST DEFINITELY not pilot's discretion on what flap setting to use. Every QRH I can think of that relates to a single engine approach and landing dictates what flap setting to use. The reason you don't use full flaps on landing during a SE app. and landing is that there is always that chance you might have to go missed. There is just too much drag associated with full flaps to even attempt to go around on one engine and meet all associated climb gradient restrictions.
I couldn't have said it better myself. For all airlines, there are set procedures that dictate when to do what.
On the single engine reverse thrust issue: Most do not prohibit the usage of it. Many do not even recommend not using it. It is usually worded in a way to question whether its use is necessary. Keep in mind, there are many many engine/aircraft combinations here. At my airline, upon landing single engine, the QRH is worded "If required, use remaining thrust reverser carefully upon landing." In my experience, 1 thrust reverser isn't going to decelerate you much, but it isn't going to twist you much unless you're on a slippery surface. Usually, when I was landing single engine, I'd just crack the remaining engine into idle reverse just to keep it from pushing me along.
On the flap issue, same thing goes, especially in an emergency/unusual situation. The QRH is the bible, and the word of the lord (read: FAA/Aircraft manufacturer/Airline). If it says to do something, you do it, unless you truely believe, deep down in your heart, that you have thought of something that all of those people missed.
As was stated, single engine usually has a lower flap setting and higher airspeed just due to the possibility of a go-around (ie flaps just have to travel less). In all CRJs, normal landing flap setting is 45, with immediate selection of Flaps 8 on go-around. Single engine landing is flaps 20, with immediate selection of Flaps 8 on go-around. The flap lever is actually built with this in mind. I know the newer (less than 20 years old ish?) Boeings have this as well in 2 different spots. There is a detent on the flap lever, requiring a change in motion to get past it. This basically allows the pilot to smack the lever, and it will travel to the detent and stop. (sorry, that's a bad way to explain it. Should I elaborate?)