1. Is it safe to ship the hardware wallet to my cousin in the UK and he will send it back to me in Nigeria or this is a good idea? I read that it's safer to buy from official than going to third-party store, I don't want something malicious inserted in the hardware wallet, waiting for my first deposit into the wallet.
It should be sure if he makes a purchase for hardware wallet from official shop or from UK resellers, and then send you the unopened package.
Anything can happen in customs, but you can always confirm if wallet is authentic or not, and generate brand new seed words for your wallet.
You can also make your own alternative DIY signing device (krux, seegsigner, diy trezor), or you can use old smartphone as offline wallet (if you remove sim card and turn on airplane mode).