The spin luff should be kept pretty tight. A general guide to check the tension is to set the spin and grip the luff in your hand. With proper tension you should only be able to rotate your hand about 90 degrees.
As for the tack line, it should stay put. I've seen people ease the hailyard a few inches in certain conditions but 99.9% of the time the tack and hailyard should remain fixed.
As for the main, you shouldn't need to dump it as far as you are describing. Sailing downwind without the spin and just dumping the main is begging to pitchpole. It might sound scary, but you will be more in control of the boat in heavy winds with the spin up versus keeping it down and dumping the main.
Well....true to a point. Water conditions play a roll but typically if it's over 30 you'll never survive with the spin up.
I'm still not sure I follow you with your mainsheet length. With the traveler all the way out, you probably shouldn't be able to sheet out enough to hit the daggerboard. I seem to remember it being close, but not far enough. Over time, I tend to shorten my mainsheet as much as possible so there's not so much slack to manage. When trying to round up in those conditions, it's important to manage your speed and try and gain some before making the turn. It's a little counter-intuitive but if you pick up speed, your apparent wind moves further forward and then you can make your turn without the wind hammering you at 90 degrees. Typically, though, it's the bear away in those conditions that's so tough.
