When being in a perfect "Long Valley" situation, does momentum help?
The answer is simple: in a perfect situation never. These optimizers are 1st order methods (unlike for example Hessian-Free optimization). The reason for they escaping from the stationary point above is that there is some small gradient (too small for ordinary SGD) in the "correct" direction. Momentum and Nesterov gradient can build up the velocity over time in that direction and Adagrad/Adadelta/RMS can immediately scale the gradient (increase it in the direction for which the past gradients were small).