the overflow pipe would not collapse at the slight kink and cause the system to be over pressurised as the high pressure would open the kink (unless mechanically held), It may prevent coolant being sucked back in but that is not the problem. Heralds work fine without the expansion bottle, they just expel a little water if full, then when cool down it looks a bit empty but it expands again to fill when hot, so it should work ok even without the overflow bottle/pipe