In an email template, can we set up in the footer section a single one-click-to-unsubscribe link, which would serve as Master Unsubscribe at the BU level?
Can we set up a Cloud Page with a SmartCapture form to serve as a preference center? Can the latter be set up without using gazillions of lines of AMPscript code?
We're using data extensions for send-outs and the AllSubscribers is most often used as default Publication List. I saw some questions arose on this topic but, honestly, I've been having difficulties comprehending them, let alone implementing them in our SFMC.
I don't believe you are going to have much luck implementing this using a smart capture form on a CloudPage. You are going to have to execute a logUnsubEvent in order to update a subscriber's status within the platform, but the smart capture form is made to capture data and not execute functions on it (without some scripting anyway).
So, in order to accomplish #1 of your question above, you are going to have to code that solution up yourself. Though there is some ampscript involved, it's not actually that complex. Here's an example script from the SFMC documentation that should give you all you need to get going. Just pass the fields it's asking for with the RequestParameter function from you email and the logic of the script should do all the other lifting.