When I lease my Waves with the new wallet, will I still get WBTC rewards (from Byteball sales) and other airdropped assets? Or will the receiver of the lease get those assets?
Yes, you still receive them. Your waves never leave your wallet when you lease them.
The only assets you lose is Miner Reward but your pool may return them to you.
You don't receive dividends for holding Waves, you get them for holding WCT my dudes, just one of the many reasons why WCT are a superior token.