The keyboard shortcut list you referenced does not seem to be updated. The keyboard shortcut for changing the view to the mockups view seems to be ⌘+1 and not the one in the new version og balsamiq (⌘+⇧ + M).
However these are my thoughs on how it could be solved. Not sure which one is the best solution.
I have no good suggestion for the symbol view since since ⌘+⇧ + S seems to be taken obviously by “Save as”. Though one could argue if there is a need for a keyboard shortcut for “Saving as” since it is not so frequently used. At least it is not as important and heavily used as switching to symbols view.
Also if ⌘+⇧ + S would be used as shortcut to switch to the symbols view then also the ⌘+⇧ + A should be used for switching to the Assets view. This keyboard shortcut seems to be taken by “Deselect All” command. An option here is to change the ⌘+A keyboard shortcut from “Select All” to “Select All/Deselect All”. If not all is selected the shortcut runs the “Select All” command or if all is selected the shortcut will “Deselect All”. But maybe this is not the most perfect soultion.
Another thought would be to use the ⌘+1, ⌘+2, ⌘+3 for “Switch view to Mockups”, “Switch view to Assets”, “Switch view to Symbols”. But then the current shortcuts for ⌘+2, ⌘+3 would have to be moved (which are “Lock the selected controls” and “Unlock all locked controls”).
A third alternative would be to make the ⌘+M a toggle command which would switch to Mockups view when the user is not in the Mockups view and when the user is in the Mockups view it would switch back to whatever view was used before switching to Mockups view. This would acually be pretty nice since it allows you to work with two views and switch back and forward between them very fast just tho check the looks (at least very useful when editing a symbol and checking the result the changes in the whole picture).
The fourth alternative would I guess to be setting the keyboard shortcut to ⌘+ B which is the first available keyboard shortcut letter from the word Symbols. However this is not the most obvious shortcut name for a new user to remember.