Stepping mash temps

I'm contemplating a HERMS system and I have a couple of questions that I haven't seen answers to:

If I want to warm the mash by pumping wort through some kind of heat exchange (like a HERMS), how hot is it safe to make the wort? A couple of degrees higher than my desired temp? If I warm it substantially (like say 150 when I'm still doing a protein rest at 120) will I denature the enzymes doing protein conversion? If I warm it to 170 will I denature all the enzymes?

I'd like to be able to step the temperature up quickly, but I don't want to denature everything that goes through the HERMS. Speaking of which, isn't that exactly what a decoction does? Why doesn't that denature all the enzymes in the part that is heated? Is the assumption that the enzymes left in the rest of the mash are sufficient to complete the mash? Or are we assuming that enzymes don't denature right away and that the time spent at high temperature isn't sufficient to cause a problem?

Your temps will be closer than 20 degrees when holding temp. Well, if they aren't you need to build a better hx and look into some insulation.

As for stepping, the enzymes need time in addition to heat to denature. I think it would be hard to finely control a step with a HERMS system. I'm not saying it can't be done. I guarantee someone here does it. It might be easier to add boiling water to step. Lots of software options will calculate how much to add. You could always look at RIMS too. I think you can step from 120 to 150 quickly with even a small element.