Dave,
If you are seeing scattered whitecaps and no significant spray then you are probably pretty close on your estimates. I think you may be underestimating a bit based on your measured boat speeds, but you are not grossly underestimating the wind speed.
You should
always check the water level in the ballast tank before sailing. Low ballast is almost as dangerous as no ballast. You don't want the water in the tank to be sloshing side to side.
The only published reference we have for the proper mast rake is based on the mast to deck angle. That is where you need to measure it. The 4 degree suggestion is about as far back as you would ever want it to be.
Based on your description of the way the boat handles I would suspect that the forestay is too long. This would give you additional rake and make the boat round up easier.
However, you have said that you struggle to attach the forestay when you are raising the mast. That would make me suspect that the forestay is too short. I really dislike contradictory evidence.
Forgive me if you've already mentioned this, but how much jib do you have out when you are rounding up?
The first order of business is to find out why you are heeling so much and rounding up in light wind. They are probably related.
Once everything is set up correctly you should be able to sail your "X" comfortably up to around 20 knots of wind. You will put in your first reef somewhere between 10 and 15 knots. The second reef goes in closer to 20 knots. If the main is reefed appropriately you should be able to keep the boat close to 15 degrees of heel without rounding up in those conditions. A few months ago I sailed around six hours in small craft advisory conditions (20-33 knots, 4 foot waves) coming home from a multi-day trip. The only thing that was difficult was striking the main when I got back to the channel.