As I am sure you know, airlines have complex ways of determining what fare classes get left open on what days, from what cities, during what times based again on a ton of factors not the least of which is historic records of that flight. So with that said-they still mess up and oversell flights, the only problem is you can't predict it.

So to answer your Q's:
1) Don't know. Las Vegas is the one thorn in airlines sides for bumping because it plays by different rules than most other cities. LAS has a wierd traffic pattern with conventions and the bad part is that a lot of the convention traffic are trade show attendees-passengers who book travel on their own dime looking for a low fare but who can, and often do, stay over extra nights causing the airlines to anticipate a higher "no-show" rate for low-fare passengers. This works against them when they only have vacationing low-fare passengers who all show up on that flight and suddenly they are bumping double-digit figures of people.

Sundays also are tough to gauge, because although it is the busiest day to leave LAS, as most weekend vacationers are heading home, airlines don't have as many seats open for sale as the date approaches so you get that 9-7-1-0-0-0-0-0 in itn. This doesn't guarantee the flight is oversold but that it won't take any more discount flyers-LAS is notorious for them.

2) Sometimes, but not often. Certain airlines can substitue A3XX aircraft, while others can switch 757/767's or swap different versions or configurations of 737 with ease, but understand that this is less likely to occur then a simple passenger "bump" as it can screw up flights down the line and isn't the easiest thing from the logistics side.

ALSO) I am not sure how NW does their bumps. As a United guy I do know that if a Premier member is put on a volunteer list they go to the top regardless of when they are put on. If NW does the same for NW/CO/DL elites I don't know, but it isn't all that often that elites dash out to be delayed for a travel credit or a free flight (except us FT'ers) so being on the list early is the best advice I can give.