Is the level gains on Toughness accumulative?
No. See page 26 for examples which make it clear that each level is its own thing, not cumulative with previous levels. The user gets to pick which level to use it at (though that's not applicable for Toughness, since it has no cooldown).
Edit: there's also this passage:
pg 26 wrote:
A hero can activate his powers spending the required activity (section
6.5, page 24), selecting the required Soulrank level. In fact, all Power
cards have minimum Soulrank Level (the first Roman Number on the
left side) required to acquire and use that power. Some allow the power
to become stronger by using a higher Soulrank Level. As the character
gains more Soulranks, the player may choose a higher Soulrank Level
of the Power card as long as the selected level of the Power is equal to or
lower than the current Soulrank of the hero. If so, simply read the base
effect of the power and then add the additional effects/values of the
chosen level (as explained in the following examples).