Man, you do realize that most of these questions can be answered by quick Google searches, right?
Anyway, the answers to some that could potentially be hard to search:
4. Which of them is safer? Or cannot easily be hacked, all things being equal, that is all normal security instructions well adhered to? Can a hardware wallet actually be hacked elsewhere?
Your hardware wallet can be hacked through unsafe storage of your recovery seed(e.g. storing them digitally/online), or possibly if your device is stolen(though most likely very difficult), especially with a unsecure PIN. Wirelessly though? I don't think so.
6. Do you know any legit seller? If yes, please name them.
Their official websites:
Is it possible for the manufacturer or seller to manipulate the device to still your coins?
Possible. Would they? Probably not, as their reputation is on the line. But there's definitely some trust involved here. If you want to be like ultra/maximum safety, a
correctly and securely made paper wallet is probably the best choice. Readers, feel free to correct me.
9. If you misplaced the hard wallet device, must you buy a new device before you can recover your coins?
You can either use any wallet that supports BIP39, or more preferably, purchase another hardware wallet.