Increasing Devotion creates 5 tokens, or 10 tokens if it is cast from the graveyard. While the "normal" way to cast this from the graveyard is with its own flashback ability, do I still get 10 tokens if I cast it from the graveyard in some other way, for example with Yawgmoth's Will?
Yes, you get 10 tokens if you cast it from the graveyard in any way.
If the alternative effect only happened on casting the spell through flashback, the card would say so by making an explicit reference to its flashback ability. There are no flashback cards that do so, but presumably it might read "if you cast this spell for its flashback cost", similar to the wording of the Mutate ability.
Since it doesn't make a reference to flashback, it happens if you cast it from your graveyard in any way. There are several more flashback cards that use the same template.