Making an Edit button in a content type form

Question: Is there a way to open a form in Edit mode using a Javascript button and a function? I'm trying to allow users to toggle back and forth between Display and Edit in a content type form. I can make the Display mode button by making a Save and Submit button and assigning the Redirect URL to  {Common:ItemUrl} 

Is there something similar that will allow me to access Edit mode? 

Background: I know how to do this on form that is local to a list by using something like the following in the Redirect:'{ItemProperty:ID} &mode=1','_self');
This solution won't work because I want to use a Content Type to control my Classic form. 
I will be using this form in seven different sub-sites and if changes are needed, it made sense to me to be able to propagate all changes with a content type. I have discovered that you cannot edit a form on a list without breaking the inheritance of the content type (the form appears to become local to the list if you edit it), so I can't add a relative URL at the list level. Something like {Common:ItemUrl} but which opens the form in Edit should work. 
I guess a corollary to this question is, is it a terrible idea to use a Nintex form with a content type? Nintex forms seems to handle content types quite differently than does a standard SharePoint form, and I haven't used the two together before. 
Thanks for any advice! 
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