I think option 2 is most likely the best route by extending the 2nd 6+2 to another riser so you are under the recommended Watts. There may be different opinions here. Maybe also consider Molex to 6 pin to power a riser or two.
The last statement from the above post would be my recommendation as well.
While you can in theory run the cards off the 6+2 cable if you spread the current over more wires using your unused molex lines that would be the best option.
Most consumer PSU cables even with the 300 watt maximum rating are meant for maybe a few minutes peak current at 300 watts, not 24/7/365 at those levels.
A more realistic continuous load would be around 200 watts, so as you said yourself it would be pushing it. If you had 16 gauge cables maybe, but with 18 gauge I would spread the current around across a few more cables if you can.
Another $10 invested in a couple of Molex to 6-pin adapter cables will give you peace of mind that you won't burn your house down as well as probably providing your rig increased long-term stability.