TV/Radio/Offline marketing can only be attributed via these statistical models (eg in the zip codes where the ads ran, did a bump in traffic or store visits happen). Although old fashioned, I think coupons per channel are a great way to get confirmation of the link between ad and purchase, it's less precise than digital tracking, but at least makes some concrete connections.
Multi-device tracking without a login can only be done by using a huge network of 3rd party data. E.g. if xyz company has cookies on everyone and they see a login on one site, then they equate that cookie as the same user across devices, effectively sharing the information with their clients. I think this is very shady and I do not use any data like this because I think it violates people expectation of privacy across interactions.