on 1: if you put more than about 70 items the selection manager it really starts to degrade in performance, at least in my experience.
On 2: You could use the load case manager to use linear superposition to solve only primary load cases and then sum up combinations there of.
On 1: I'm glad to know it's not just me.
On 2: I considered the load case manager, but it is not included in my license of Simulation. Also, my FEA needs large-deflections and no-penetration-contacts. I suspect that load case manager (with linear superposition) will not work with these.
On 1: welcome to the club.
On 2: You just use the one study and just change the loads there........also the design study might be a way to get it done.
Here are some comments/suggestions:
1) You can copy loads/parts/shells/meshes from 1 study to another by selecting them in the simulation tree and dragging them to another study tab. This way you can change shell thicknesses once and drag to the other studies assuming your overall geometry hasn't changed.
2) I use the API extensively for things like this. If you are not familiar with it it is definitely the most difficult option but the most powerful. It uses a programming language such as Visual Basic to automate simulation functions. You can actually put all your thicknesses in an Excel spreadsheet and drive the simulation model from there. Start by going to menu Help->API Help.
3) General tip: If you name your surfaces in the modeler with the thickness, e.g. "TopPlate.375" for 3/8" you have the thickness in front of you while in the simulation tree. Doesn't populate thicknesses but just a handy "reference" when you're manually entering thicknesses.
4) I'm not familiar with sheet metal features but I believe those are "surfaces" that have a defined thickness going into the simulation. Of course this would change your whole modeling approach but there it is.
Hope that helps.
"Import Study Features" is very useful if any of those shell definitions happen to occur in multiple instances or a pattern in an assembly
I used this recently on a fairly complex shell model and it saved a tremendous amount of time. Define the shell, material, mesh controls, contacts on a study of the child part/subassembly and then import it to the top level study