The worst part about gutter oil is that the "reprocessing" actually makes it more dangerous. The worst chemicals aren't broken down by heat and become more potent as the other garbage (literally) boils off.
The hotels will change cash, but only perfect foreign bills. I got told "this bill is broken" when it had a small crease in the corner. Either use an ATM in China or use one in HK and have Travelex do the conversion. There are also ATMs at the Hong Kong airport that dispense RMB. I trust those more than the ones in China. Beware that Travelex will convert your foreign currency to HKD and then to RMB if you bring them cash, doubling their fees. Use the ATM.
It's much easier to take a cab from HKG to the border and walk across if it's evening time. It's about $300HK/ $50 USD and you can take 1-4 people. The Shenzhen Bay crossing closes ~11:30PM, though, so if you're late you'll have to find another way.
During the day, a ferry is a better bet because customs can be extremely clogged up at the walk through checkpoint. I've waited 2-3 hours to walk back across into Hong Kong. There are far fewer people at the ferry terminal checkpoints.