Mystic mine vs Denko Sekka
Mystic Mine only prevents the activation of monster effects for the player who controls more monsters.

Continuous effects, such as the effect of Denko Sekka, are simply active as long as the card is face-up on the field.

Moreover, Majesty's Fiend imposes the same restriction as Mystic Mine. In all three cases named, there are no activated effects. Therefore, yes - Denko Sekka and Majesty's Fiend will both be applying their effects in each of the three cases.