"...the only advantage a restrained guard receives is counter strike....."
Yeah, that part's tricky. The other day, I saw someone get caught up on that text; he thought it meant that all restrained creatures got counter strike.
So that sentence has come up on the forums before.
I read it as explanation of which parts of the guard rule a guarding creature gets, not a blanket rule forbidding any benefit from guarding, but I can see how it might be read as a limiting rule.
Basically, I read that section as saying, "When you guard, you get to force attacks to the guards (protect your zone), and you get to counter strike. Unless you're restrained, then you only get to counter strike." But, again, I understand that this may be the softest part of my argument.
What I think is more robust, though, is that I don't think that intercept actually IS an advantage of guard. Intercept is its own ability that relies on it's own ability word, it's own codex rules, and a guard marker (which we both agree is present on a restrained guard), but doesn't actually depend on ANY of the other rules for Guard. Guard could be a blank and useless rule, and as long as it put a marker on the Dwarf, I think Intercept would still work. If there was some other way to put a guard marker on a creature without using the guard ability, Intercept would work with that, too.