To your first question, It is as same as how you will get the subscriber key from salesforce via normal email using personalization strings. You can use
_subscriberKey for salesforce ID and
emailaddr for email address and add it in hidden attributes.
To your second question, It is possible. Because if you look at the HTML part of Interactive email form in the code view, you will find ampscripts on re-directing to cloud pages using CloudPageURL function.
So, you can use ampscript in Cloud Page / Landing page to get the subscriber key here as well. Once you get the subscriber key then you know which data extension to lookup with and get the results and update salesforce using
Set @subscriberkey = [_subscriberkey]
Below is the screenshot if it: