I have almost the exact situation as the original poster, except, instead of the cross wind across bow quarter at 45 degrees, it's across the stern quarter. My slip is 14 ft. wide, and the beam of my C320 is 11' 9". Slip is accessed from a 50 ft. wide fairway. There is no current to contend with. The C310 and C320 are extremely similar in design.
Your comment about docking with 30Kt cross wind and 3-4 current is interesting. You want to secure the stern line first, which implies that the boat needs to be almost completely inside the slip before you step off to secure the stern line. Are you docking in a traditional "rectangular" shaped slip and how wide is it? The 30 kt wind is going to give you significant drift across the slip and into the adjacent boat in the OP's diagram. Or is the wind setting you onto the finger pier in your case? I read about these types of landings, but I can't do it and I don't understand how others can do it. There's something major that I'm missing and I hope you can enlighten me. Last year, I did some testing in the slip regarding drift/set from the cross wind and I determined that I had on the order of 2 seconds in 25 mph winds from the time I entered the slip before I was going to contact the leeward outer piling. And I wasn't going to be very far into the slip, so the wind would then tend to twist the boat back into the fairway if I stop, or otherwise, I'm going to damage the boat if I continue to try to go further into the slip, or if I can't get it stopped when it strikes the piling. Also in OP and my situation, the backing into the slip before the actual turn into the slip is somewhat down wind and direction of propwalk is going to combine to spin the boat around out of control when one makes the backing turn into the slip (or wind alone, if you shift to neutral for the turn). Further backing with a 30 kt cross wind down wind means you have to be really moving in reverse.to have control over the boat. Miss judge the turn just a little or not getting the boat stopped in such a hot approach is going to create a whole world of other problems. What am I missing? When I do all this, I will be single handing.