First of all, if you are getting grains in your boil kettle, it is a sign that your lauter is not working correctly. You not only aim to filter the wort FROM the grain, you also aim to filter THROUGH the grain. You want to get as much goodies as you can by rinsing the grain bed with water. I imagine it might not be working so well if it sucks the grains into the boil kettle.
Secondly, I never read that boiling your mash is a bad thing. On the contrary, as you point out, in decoction mashes it works perfectly. The only thing you need to look out for is that you do not denature your enzymes too much.
If I am not mistaken, that is why you try to take out the grain, rather than the liquid part of the wort for the boil. The enzymes should be dissolved in the liquid.
Last things is the tannin extraction. That is why you do not SPARGE with water over cca 78°C, but great deal of it depends on the pH. If you adjust your sparge water to be of the same pH as your mash, sparging with water over 78°C will not hurt so much.