Hi Andy, your interpretation of the intention re last light is correct. You must PLAN to be on the ground 10 mins before last light. By law you do not have to be on the ground 10 mins before last light.
However, if a circuit takes about 6 minutes or so, that 10 min buffer only gives you one go around. There are also alternate minima criteria based on forecasts.
Having said that, having your own personal limit above the regulations is a good idea. Perhaps your own personal minima might be to allow sufficient time for a diversion. I wouldn't like to plan to arrive only 10 mins before last light unless it was a short flight on a sparkling clear CAVOK day, or if I had a current night VFR or instrument rating. It would pay to give yourself a bit more time so you are not caught out.
On the circuit entry, the ruling states that IF you choose to overfly, it should be 500 ft above the ACTIVE circuit. So if you are flying to a country strip with no RFDS, RPT or high performance aircraft, you can overfly 500' above circuit height. Even if the airport has RPT etc, you can still overfly at 500' above your circuit height if there are no high performance aircraft at the time. You'll know that from looking and listening. Look for high performance traffic in the high circuit, and listen out for any radio calls. RPT pilots make mistakes too, don't assume they will be on frequency so do look out. If unsure, you can overfly 1000' above, but remember a quick descent at low power settings isn't that great for the engine.
Remember too that you don't NEED to overfly either. If you can establish the circuit direction in use or you have a good indication of wind (eg AWIS, local smoke etc), you can join directly on crosswind, downwind, base, final etc. If you are not sure though, do fly over and check the windsock.