I'd like to answer your questions in order:
>> How do you hide the expand/collapse button (the one at top with the 3 lines in it)?
For this purpose, it should be sufficient to set HamburgerMenu's HeaderHeight to 0.
>> How do you keep the menu expanded at all times and not allow collapsing?
HamburgerMenu supports adaptive layout and changes its state automatically. You can specify what states are available in this control. For example, if you set the AvailableViewStates property to Inline, your menu will use only this state. See also: Adaptive Layout.
Let me know if you have additional questions.