It's ok, we have all been there!
Congrats with the retirement, I hear its more work than work...lol. Ok, lets look at your situation and see if I can put out any of your fires.
1. The reason I have found for the 3 horse popularity vs 2 is the versatility of the extra stall, ie. A place to put hay, leave the third stall open for more room, bring a friend, you name it its been done in the third stall. The weight difference for the additional stall averages somewhere between 500 - 800 lbs depending on type of trailer and the materials its made of, ie. High side (stock style) vs double wall (fully enclosed) and aluminum vs steel, etc. When you figure adding that weight to a bumper pull it can account for as much as 20 -25% more vs a living quarters only adding 5 - 10 % more in weight.
2. For a living quarters....YES, you will need something a bit larger. Now, that doesn't mean you have to buy a diesel dually. You would be smart to consider both purchases, the truck and trailer, at the same time to maximize bang for the buck. To make it simple, when you think have the trailer you want figured out you can get a weight from the dealer and start your truck shopping knowing what you will be towing.
3. Slant loads come in all shapes and sizes, from 6'6" wide to 8' wide and the same goes for the height. Most companies offer warm blood packages that incorporate the popular warm blood options the people before you wanted and requested. If your having trouble getting answers from your local dealer, I suggest measuring your warm blood trailer now for a reference and applying those measurements to the slant loads you look at.
4. As far as choosing a manufacturer, it couldn't be any easier these days with the internet being what it is....a wealth of knowledge. Now, I'm saying do all your homework here, just a little of the leg work. I always tell people when looking for a trailer manufacturer to look for a couple things. The first thing you are going to need to consider, especially with a living quarters, is the proximity of you to the dealer you are purchasing your trailer from. You will more than likely need to visit them a time or two, just count on that. Living quarters are essentially houses on wheels and require a host of maintenance and upkeep. You can have most work done at an RV dealership at a hefty hourly rate, but if it is warranty work you will want to be close to your dealer so they can provide you with service after the sale. A lot of problems are little things that are common to that brand and the dealer may know how to fix it in 30 min as where an RV dealership may have to start from A and not figure it out until Y and a three figure bill!
Another thing to consider, this is where the internet works wonders, is the reputation of the manufacturer. Google is your friend, Google the brand and the model you are interested in and read the feed back people before you have. Check the Better Business Bureau (BBB.org) for the manufacturers name and address for complaints and resolutions. I have really enjoyed going to local equine events and just walking up to people unloading their horses and asking them what they like and dislike about their trailer, most are happy to talk about them and even give tours!
5. This is probably your most controversial question asked! I try to keep it simple with a few facts.
A. Cost to repair: (Steel<Aluminum)<Steel/Aluminum. This little formula represents the fact that steel is the most common to repair and more people can do the job which brings the price down. Aluminum is the most per hour in welding and replacing of parts due to the complexity of welding aluminum properly. The steel/aluminum configuration usually has the most man hours in fixing due to the way the aluminum is attached to the steel and you have to remove the aluminum to repair the steel and that in itself can be a chore. These scenarios aren't always the case, but often are.
B. Weight: This gap isn't near as big as a lot of people imagine it to be, especially in a living quarters. Small bumper pull trailers are really the only ones that benefit in the weight department here. In a living quarters aluminum requires more reinforcement and or usually thicker material to hold up to what steel can carry. When it comes to the steel frame/aluminum highbreds, they usually use thicker aluminum on the walls to withstand dents and in turn come close to washing in weight. The biggest reason manufacturers use aluminum on a steel frame is for the speed to finish and the durability of the exterior finish vs a painted steel. The thing to watch for is the steel frame prep before the aluminum is applied because you could be looking at dissimilar metal corrosion down the road if not done properly.
This section can take pages alone, so I will sum it up. The above isn't always the case, there are always exceptions to the rule. You need to find a manufacturer with dealers in your area. You need to check their reputation (dealer and manufacturer). Compare the different models and the weights to see what works best for you. Keep in mind the cheapest in the short term, isn't always the cheapest in the long term.
I hope that helps some. I will be reviewing trailers on another site in the near future, so if you have a trailer you would like me to look at, send me a pm and I'll see what I can do.