You have to use an alternate delimiter, there is no method to escape it.

The best way to capture the mid is per the documentation here:https://marketing.adobe.com/resources/help/en_US/mcvid/mcvid-getmcvid.htmlvar visitor = Visitor.getInstance("YOURID@AdobeOrg") var analyticsID = visitor.getMarketingCloudVisitorID() s.prop1=analyticsID;  CM Edit: Added hyperlink

 _satellite.setCookie('source','ELEMENT VALUE',1) should work fine

Your best bet is to fire a rule based on a data element that is common across all those 100 websites.  That data point might be url structure, data layer attribute, etc.You can also create a script that lists all the domains you wish to track which evaluates to true if it matches the current hostnam...