1/ phone tethering is the price to pay for end-to-end encryption: the support of multiple devices is not compatible with perfect forward secrecy, as the former require the asynchronous push of messages to all devices while PFS requires synchronicity (at least some kind of, as explained in their white paper here: https://lobste.rs/s/sx2f0r/whatsapp_encryption_overview_tech...
No it doesn't. Multiple devices is a special case of multiple recipients, which the Signal protocol handles (and I presume Whatsapp does as well). See more details here (https://whispersystems.org/blog/private-groups/)
Now, that doesn't mean it already works in Signal/Whatsapp, because they have other constraints. But it's not PFS that prevents it.
2/ if you don't like the Desktop App (or are using Linux), you can use https://web.whatsapp.com/ and/or the Chrome extension WhatsChrome https://chrome.google.com/webstore/detail/whatschrome/bgkodf...