I didn't read all of the responses in this thread, but protein is a requirement for me. I stretched for 32 weeks, and if I didn't use protein at least once a month during my stretch, my hair would have been breaking like crazy. This year, I am stretching for 12 months, and I couldn't imagine doing it without some form of protein.
Protein is important for me when I stretch because it strengthens my hair and helps my hair retain moisture
better and longer. It actually made my hair softer when it was dry (airdried). When I needed protein, my hair would be mushy and soft when wet and dry and breaking when dry. I could put half a bottle of moisturizer on my head and it would be dry dry dry two hours later
Oh, and dcing with a conditioner just made it worse (made my hair more mushy and soft). I
don't really know how it works (maybe Gym can jump in here) but from what I learned, protein strengthens the hair strands allowing it to hold moisture. It your hair is weak (especially due to frequent chemical services, over-use of heat, or from being over-moisturized) it will not be able to hold the moisture you apply to it.
I would use Aphogee or do a henna treatment, depending on the degree of the breakage, which wasn't much. I used Nexxus twice a month for preventative purposes.
But I can only speak for my experiences and my hair. The two most important things that determines how long you can stretch, IMO, are the products you use and how you handle your hair. Once you find the right moisture-protein balance that reduces breakage, you are good to go!
Also, the difference between shedding and breakage is that with shedding, the hair has a little bulb you feel on the end, but breakage does not. While stretching, breakage at the demarcation line can be a problem because of the different textures in one strand. This is why protein is important because it strengthens the point where both of your textures meet. I also suggest being very very gentle to your hair, especially when it is wet, because that is when it is most vulnerable.
Hope this helps!! Good luck on your stretch.