Hi Miller W

1 - When you draw 2 or more walls and you join them using the mouse, you will notice that they will "fuse", and instead of two or three different walls you will have a set of walls, with the same properties (thickness included)
However, if you join them using the arrows (keyboard) the walls will be together, but they will not fuse, therefore they will not share their properties, so you can use different thickness
The exterior walls can be fused only with other wall in the starting point and other wall at the end point. This is why, after you create a closed perimeter with the exterior walls, you can join the interior walls with the mouse, and they will not be fused with the exterior walls

2 -
Some resources
--> Try to develop your work in 2D and then go to 3D, so Punch doesn't need to render all the model with each modification
--> You can turn off the "Undo" feature--> Edit--> Undo preferences. This will speed your work. Just remember to save frequently because, in this case, of course, there is not "Undo"
--> Check your computer for adware, spyware and such. Some users experienced "crash issues", computer slowness and error messages caused by these programs
Lavasoft is an excellent (and free) application to clean you computer
http://www.lavasoftusa.com/
--> To get a good Punch performance, close other programs, including the e-mail. Perhaps, other programs are using resources otherwise required in Punch.
To verify your CPU resources--> Close all the programs--> Click Ctrl+Alt+Del in your keyboard--> check the Performance value, it should be a very low value. If not, check the Processes tab and verify where the resources are going (check high values)--> Then open Punch--> Repeat the Ctrl+Alt+Del process and verify the CPU usage and processes--> This will provide a good piece of information if the problem is with the file or with the computer or both
--> Don't forget to defrag your HD, it will speed your performance

Hope this helps

Patricia