Overflow considerations...

I don't believe that the 9" has anything to do with the number of slots or the width of the slots, but it is referring to the area that should be exposed to the surface for best  surface skimming (i.e. 9" overflow would be 3" wide with 3" front to back depth or 5" wide with 2" front to back depth), not with the drain capacity itself.
The recommended way to break the siphon with the return feed is to drill an anti-siphon hole in your return line that's above the water level of the overflow, but can be within the overflow itself.  If your return line is not inside your overflow, it should be just under the surface of the display tank waterline.  This anti-siphon hole should be regularly cleaned as slime, calcification, algae or detritus can clog it and prevent it from breaking the siphon.  As a general rule, I assume that mine is always blocked and I don't place corals above the water level that my tank would drain to if the anti-siphon hole were to be blocked.  I consider anything above the return water outlets to be a potential dead zone.  And without an anti-siphon hole, the highest water outlet is the level at which the water siphon would break.  Think of it this way...how much water is going to drain before air can get into your return line.  The air is what breaks the siphon as it can move through the plumbing with less resistance than the water.
As for placement of the return lines, that can be done a million different ways.  The best way is a method that would allow it to disburse the return water around the tank using a device like a sea squirt or something like that.  That being said, most people (myself included) just use loc-line and point a couple of outlets to different places in the tank from a single return line.
Remember, don't count on your return line to create your flow unless you're doing something like a seahorse tank (very low flow).  You will use powerheads inside the tank to create your in-tank flow.  The return pump should be used primarily for water exchange between the display and sump.
These are great questions which are excellent questions to ask on a tank tour as you can ask people why they have their tank set up the way they do and you can see physical representations of these answers vs. just a text explanation.
--AJ

 

You will loose pressure/efficiency with an anti-siphon hole, but it's what you should do.  I don't know how much you loose, but it only makes sense that it would impact performance.  A Mag 9.5 is ok for that.  When you take into consideration head loss, at 6 foot head pressure, you're down to 700 GPH.  That's a single pipe, straight up.  Add bends, curves, T's, etc and each time you cut the line and add in a fitting of some sort, you impact the flow.  I'm not sure that going to just your tank will give you the full 600 GPH, much less putting in a T and running some back to your sump.  If that's really important to you, get a Mag 12 or larger.
Here's the performance curve for the Mag 9.5 pump:

I can't comment on the MaxiJets because I don't use them for flow in my tanks...all I use them for are utility pumps to feed reactors and stuff like that.
If you want to properly size your drain/return line, look at the sizes of plumbing that the manufacturer uses for that size tank.  I think for a 90, they use 1" drain and 3/4" return...thinwall PVC.
--AJ

 

For most purposes, I like thread / thread bulkheads, but IMO, for bulkheads inside of overflow boxes, thread/slip are better - at least for the drain side.  The reason for this is that frequently you don't have a lot of room to turn the plumbing and thread the plumbing into the bulkhead.  For this same reason, DO NOT GLUE your Durso standpipe together - at least not the elbow that's going to be partially submersed.  I also prefer to use a "street" elbow (female/male elbow - most of the elbows are female/female) for this vs. a regular elbow as they take up less room.  If you glue this together, you may not be able to get it installed.
This is typically not as much of a problem on the return side as it is on the drain side so it's OK to use threaded on that side.
I use threaded components whenever I can...especially ball valves as they are so darn expensive.  One other thing, Schedule 40 ball valves are not good over time if they
normally run open.  You will likely not be able to get them completely
closed over time, or break off the handle trying to do so.  You should
get Schedule 80 ball valves.  I know the price difference is 2 or 3
times, but you'll thank yourself later.  Also, you can get threaded unions at Lowes, so I try to use threaded unions as well. 
If you're placing a BRS order, let me know and I'll split shipping with you.  I need a few ball valves and Herman needs a check valve.
--AJ

 

The teeth on an overflow are for keeping things out of the overflow and surface skimming.  Not about flow.  If you notice on an overflow with teeth, your water level is 1/2" to 3/4" above the bottom of the teeth.  The water level will raise high enough as it needs to get the amount of water that you're pumping into the tank into the overflow.
Also, a 2" long tooth on the overflow would mean that your water would drain that low in case of a power outage.  I wouldn't recommend teeth more than 1".  Also probably no wider than 1/4".  It looks nice when the gap between the teeth is the same width as the teeth if you can manage this.  Look at commercial overflows online and see how they are rated (i.e. www.glass-holes.com has some available).  They have a 700 GPH overflow that's 2" X 6" X 6.25" tall.  Knowing those dimensions and counting the number of teeth in the picture should give you the answers you're looking for.
Of course it makes a difference in the amount of flow that an overflow can take when you have longer teeth or narrower openings.  Reef tanks are not exempt from the laws of physics. 
--AJ