On a dual monitor workstation, if I have the vm in one monitor and it is maximized and I click in the other monitor (host), the VM should remain maximized. This could be an option where the default exhibits the current behavior, something like "demaximize a maximized VM when clicking in Host window" Product: workstation for windows.
Why not to use ctrl+alt ? In theory it is possible, but there is should be balance between features and software complexity Every feature adds additional point of failure
You can change mappings at Parallels Desktop -Preferences Keyboard - reassign ctrl+alt to another mapping
