1. "Easy to backup (only need HDM Cold seed)" does this mean the Server key and the Hot key are derived from the cold key? (as if it wasn't, I would assume you'd also need the xpub from the server key and the xprv from the hot key to generate the addresses and sign for 2 of them.)
Each HDM seed is generated separately on its own device. Server key and the Hot key are NOT derived from the Cold key. The Cold key is used for HDM Server's BitID access control. In this way, if you lose your hot phone, you can use your cold phone and server to recover bitcoins, and if Bither server is down, you can use your hot and cold phone to recover bitcoins. Generally, you only need to backup the cold seed.
2. Will the user be able to scan / input their own xprv/BIP39 phrases as their key(s)? (In case I want to generate my own entropy) How would that process work?
Currently we do not support importing seeds, because importing will make the HDM account generating process too complicated, we haven't found an elegant way to do this. We only support recover cold seed from backup at this point.