I see two different issues here:
I quickly laid out my plan and then went to adjust the measurements (millimetres) when I adjusted the walls using the resize tool the walls adjusted from the middle and the walls that run at right angles skewed to the new length leaving the wall no longer at right angles.This also occurred when I use the alignment tool.
If you resize the walls (or any other element) using the arrow with the # sign (Edit Toolbar), you can choose if you wish to resize the element from the left, right or center. Otherwise, you can select one of the ends (yellow point) and move it using the arrows in your keyboard. In this way, you will keep the walls with the right angle.
According to the manual there should be a locking option available, but I cannot find it.
Excerpt from the manual
Locking Objects in Place
Many times you will want to lock an object in place after it is positioned. This avoids accidentally moving the object.
To control object locks
1 Click to select the object you want to edit.
(optional) Hold down SHIFT and click to select multiple objects.
2 On the Control toolbar, click the Lock Tool to lock the selection(s) in place.
Click the Unlock Tool to unlock the selection(s).
(alternatively) Click Edit menu > Lock or Unlock.
The User's Guide is divided in chapters according with the different subjects and tools. The names are in the top left corner.
The "Locking Objects in Place" paragraph that you are mentioning (Page 465 of the User's Guide - Chapter 35) is part of the 3D Custom Workshop Power Tool. It is not a feature of the main program.
In the image attached, you can see a screenshot of the 3D Custom Workshop Power Tool, showing the "Lock / Unlock" icons (outlined in blue).
Hope this helps.